My unused C drive capacity is at 4%. I've deleted temp files, recycle bin, internet files and compress old files. I tried to defragment but don't have enough unused capacity to run it. Any thoughts?
If not possible, free some disk space as follows:
Delete Windows ".Log" files,... empty C:\WINDOWS\Temp folder, Windows Updates Uninstallers (hiden files) in C:\WINDOWS, and install CCleaner. This application can delete several hundred MBs of crap from a smal 40-80GB Hard Drive after several months of use.
To make more space you could compress user files in RAR or ZIP folders. Also clean and defragment the registry to make a little more space using a Registry Defrag application, it should compress some 2-5% depending on the System's age.
Another option you could try if the previous suggestions aren't enough to allow a defragmentation, is to compress the drive contents to save disk space (C:\Properties\General tab). That should free some 10-15GB for every 50 GB of used disk space. You can uncompress it when the defragmentation is done. This can be done safely, the system will not malfunction while it is compressed I have done it without a problem.