C-drive keeps filling up

You may need to show hidden files. Type Folder Options in the search box above the Start button and select View, Advanced Settings and verify that the box before "Show hidden files and folders" is checked and "Hide protected operating system files" is unchecked. You may need to scroll down to see the second item. You should also make certain that the box before "Hide extensions for known file types" is not checked.

Let’s check a folder where something is obviously wrong. Your system32 folder (C;\windows\system32) should be nothing like 51.8gb. On this computer it is 4.2gb (or 4239.8mb). The larger sub-folders are:

Driver store                                     1766.2mb

2789 Files                                        1405.8mb

Config                                               381.9mb

Winevt                                              119.3mb

Drivers                                                65.4mb

Spool                                                  62.3mb

No other folders over 50mb

What are the corresponding figures on your computer?

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?

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.

    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.