I was running out of disk space in C: and was unable to install a small software that I needed, so I decided to clean up a bit. For that I like using WinDirStat http://windirstat.info/ which very quickly allows you to find where the big files/folders are. In this case I found that my c:\Windows\winsxs folder was over 12 GB of size. One way to reclaim some of that disk space is to cleanup all files that have been backed up when a Service Pack has been installed. To do that in Windows 7 you can run the following DISM command:
dism /online /cleanup-image /spsuperseded /hidesp
That freed up 4 GB in my machine and now I can move on.
Disclaimer: I only ran this in my Windows 7 machine and it worked great, have not tried it in Server SKUs so run at your own risk.