C-drive keeps filling up

Glad to hear that you've got free space again!  Now, let's see if we can figure out what is writing those files in the first place.

1. Download and run Sysinternals Process Monitor (it is a standalone application).
2. Clink cancel on its Filters dialog.
3. In the Filter menu, make sure that Enable Advanced Output and Drop Filtered Events are both ticked.
4. On the toolbar, in the group of five buttons on the end to the right, make sure that only the second one, Show File System Activity is ticked.
5. Click the Filter button on the toolbar (or press [Ctrl]+[L]) to bring up the Filters dialog.
6. You will see a row of controls at the top of the Filters dialog where you can fashion a rule.  Set them to "Path" "contains" "NetworkService\AppData" then "Include".
7. Click [Add], click [OK], and then click the Clear button on the toolbar (or press [Ctrl]+[X]).

    When a program starts reading/writing files to the Network Service AppData folder, events should start appearing in the list, along with the name of the process that is responsible for the writes.  If nothing appears, make sure that the Capture toolbar button (the third one, it looks like a magnifying glass) doesn't have an "x" over it.  It if does, click on it to enable monitoring.

    Please note that due to the rather loose filter we set, some legitimate events will probably end up in that list as well.  After running for a few minutes on my system, a couple of entries appeared pertaining to a "VirtualStore" from svchost.exe, a Windows component.  The kind of suspicious activity that is filling up that folder on your computer should be hundreds or thousands of events, so it should be obvious.  You can click on the fourth button on the toolbar (next to the Capture button) to toggle auto-scrolling on or off.

    You may be able to figure out the problem on your own by Googling the suspect's name (as long as it isn't svchost.exe).  If not, you can take a screenshot of the suspicious activity and post it here, or save the Process Monitor log by clicking Process Monitor's save button and upload it somewhere, posting a link here.

    Wow.  That's a lot of files!  It is very suspicious that it is the NetworkService's user profile that is filling up with Temporary Internet Files, and not your user profile.  This suggests that some service was accessing the Internet via Internet Explorer or an Internet Explorer ActiveX control.  Incidentally, my computer's NetworkService Temporary Internet Files folder is completely empty.

    For now, I am going to assume that any infection has been removed, and we just need to clean out its payload.  I wrote a program that is specifically designed to delete massive amounts of files, and files where the pathnames are too long, both which are scenarios that Explorer does not handle too well.  If you have not already been successful clearing out that folder, please:

1. Cancel any delete operations currently in progress.
2. Download my program: DeleteFolderTree.exe (42 KB standalone portable application).
3. Copy the path to the errant folder: C:\Windows\ServiceProfiles\NetworkService\AppData\Local\Microsoft\Windows\Temporary Internet Files\Content.IE5
4. Paste it into my program and click [Delete].

    Unlike Explorer, my program won't have a "Preparing to delete" period; it will start deleting files/folders immediately after confirmation from you.  Unfortunately, there will also be no progress indication, only a count of deleted/failed items, and a readout of the increasing amount of free disk space.  This is due to my program's skipping file enumeration, which is what took TreeSize so long, and is what Explorer is doing during the "Preparing to delete" phase.  My program does the enumeration and deletion at the same time, which means it never knows how many files are left to delete.

    @Frederik Long and HairyFool:  I know you guys were trying to be helpful, but if a user changes the Temporary Internet Files cache size setting, if will only affect their user profile.  In this case, since it is the NetworkService profiles' Temporary Internet Files cache that is getting out of hand, changing that setting won't help at all.

That program you wrote certainly did the trick as far as deleting the contents of that folder.  It took a couple of days, but I now have about 120 GB's of free space again and my computer is performing like before.  Many thanks.  

However, there's still some sort of virus/malware infecting my computer because that same folder constantly fills up with .jpg, .js, and .htm files.  I run your delete program once a day to get back 1-2 GB's of space.  At least this buys me time to get rid of whatever is infecting my computer.

Wow.  That's a lot of files!  It is very suspicious that it is the NetworkService's user profile that is filling up with Temporary Internet Files, and not your user profile.  This suggests that some service was accessing the Internet via Internet Explorer or an Internet Explorer ActiveX control.  Incidentally, my computer's NetworkService Temporary Internet Files folder is completely empty.

    For now, I am going to assume that any infection has been removed, and we just need to clean out its payload.  I wrote a program that is specifically designed to delete massive amounts of files, and files where the pathnames are too long, both which are scenarios that Explorer does not handle too well.  If you have not already been successful clearing out that folder, please:

1. Cancel any delete operations currently in progress.
2. Download my program: DeleteFolderTree.exe (42 KB standalone portable application).
3. Copy the path to the errant folder: C:\Windows\ServiceProfiles\NetworkService\AppData\Local\Microsoft\Windows\Temporary Internet Files\Content.IE5
4. Paste it into my program and click [Delete].

    Unlike Explorer, my program won't have a "Preparing to delete" period; it will start deleting files/folders immediately after confirmation from you.  Unfortunately, there will also be no progress indication, only a count of deleted/failed items, and a readout of the increasing amount of free disk space.  This is due to my program's skipping file enumeration, which is what took TreeSize so long, and is what Explorer is doing during the "Preparing to delete" phase.  My program does the enumeration and deletion at the same time, which means it never knows how many files are left to delete.

    @Frederik Long and HairyFool:  I know you guys were trying to be helpful, but if a user changes the Temporary Internet Files cache size setting, if will only affect their user profile.  In this case, since it is the NetworkService profiles' Temporary Internet Files cache that is getting out of hand, changing that setting won't help at all.

Got to Control Panel > Internet Options(that way you do not have to open IE itself)

On the Browsing History click on Settings, On the Temporary Internet Files tab check the settings. Here you define the limits which should be nothing like your experience. The limit should be hundreds on MB, mine is 250, not even 1GB (its max), and set to Automatic. It might be worth setting it to a new value just to make IE recreate the entry. 

More importantly it shows the path, where is it pointing to?