Needs more information. (model of machine)
You may want to run an integrity check on your hard drive
You can run a defrag and a check disk
Type this in the run line (Start > Run)
"chkdsk c: /f /x /r" (Press "Y" when prompted)
"cleanmgr" (Select all options)
"defrag c: -f -v"
(reboot)
Run defrag again
(reboot)