Cannot defrag my computer since installing Windows 8.1?

You should get some output when it completes.  For example this is what I get for the Analysis,

C:\WINDOWS\system32>defrag c: -a -v

Microsoft Drive Optimizer

Copyright (c) 2013 Microsoft Corp.

Invoking analysis on Windows 8 (C:)...

The operation completed successfully.

Post Defragmentation Report:

        Volume Information:

                Volume size                 = 119.99 GB

                Cluster size                = 4 KB

                Used space                  = 43.23 GB

                Free space                  = 76.75 GB

        Fragmentation:

                Total fragmented space      = 0%

                Average fragments per file  = 1.01

                Movable files and folders   = 122091

                Unmovable files and folders = 9

        Files:

                Fragmented files            = 711

                Total file fragments        = 1816

        Folders:

                Total folders               = 7637

                Fragmented folders          = 25

                Total folder fragments      = 54

        Free space:

                Free space count            = 9364

                Average free space size     = 8.39 MB

                Largest free space size     = 59.09 GB

        Master File Table (MFT):

                MFT size                    = 321.75 MB

                MFT record count            = 329471

                MFT usage                   = 100%

                Total MFT fragments         = 2

        Note: File fragments larger than 64MB are not included in

ion statistics.

        You do not need to defragment this volume.

C:\WINDOWS\system32>

A DOS Windows should always return to the Prompt (C:\WINDOWS\system32>) when it is ready for the next command after completing the initial command you entered.  You can not run multiple commands and have to wait for a command to finish executing before entering new commands.  So when it comes back to the Prompt (C:\WINDOWS\system32>) you know that job has finished.

OK so I got the results and I think it is now defragging but it's just an open command prompt window, does it tell you when it's done?

Those codes seemed to work thanks!

Same here and I just bought the machine yesterday!

I found a solution that worked for me here:

Strange Defrag error 0x80070057

http://social.technet.microsoft.com/Forums/en-US/6d782ade-f4de-43bf-8cea-647828b35757/strange-defrag-error-0x80070057?forum=winserverfiles

Open an Administrator command prompt and make use of fsutil:

fsutil usn deletejournal /D <volume pathname>

fsutil usn createjournal m=<max-value> a=<alloc-delta> <volume pathname>

Eg :

fsutil usn deletejournal /D C:

fsutil usn createjournal m=1000 a=100 C:

After using these commands I was able to defragment C: without an error showing up in the event viewer. I'm not sure if this is a permanent or temporary fix, and someone mentioned in that thread that it didn't work with a server so YMMV.