start format c:\
Install ANY linux
Sorry, I couldn't resist.
Have you checked if your temp folder is filling up? Windows does a terrible job cleaning up after it's self.
Do you have logging on? If so, what is being logged? (audited)
How much free disk space do you have available?
Are users logging off in an orderly fashion? If they are getting disconnected and the app isn't getting shut down properly?
Is this a new install?
How long has it been up?
How long has this been going on?
Has there been any new packages loaded?
Recent patches?
Run Antivirus and Adaware scans looking for anything odd running?
Hope this gives you some ideas where to look.