I tried to disk defrag my computer the other day but it came up telling me that C drive is unavailable?

i think the adminstrator is blocking ur priveledges to do defrag and do a system restore

Firstly eliminate all malware, run AV scans and download a good spyware scanner.

Some common reasons why you cant run a defrag are-

Programs interrupting its function can cause the defragmenter to restart or hang. That’s one of the drawbacks of the built in defragger, you cant defrag and use the PC simultaneously.

Disk errors, (run chkdsk before defrag)

Lack of free space , the built in defragmenter needs atleast 15% free space, for which you can try running a diskcleanup.

Fragmented system files which the Windows defragger cannot defrag neither can any of the free defraggers.

I'd suggest installing a third party tool like Diskeeper, which has many advantages like real time background defrag, system file defrag, low free space defrag and a nice GUI. Try the free 30 day trial version.

http://www.diskeeper.com/defrag.aspx

Here are 2 of the best free defrag programs for you, Try to defrag your computer with the help of one of those 2.

Bet it works

http://www.download.com/Auslogics-Disk-Defrag/3000...

http://www.snapfiles.com/reviews/iobit-smartdefrag...