Scan your computer with your antivirus software.. It may be due to a virus..
If your Drive is clean..
Downlaod & install GetFoldersize : http://www.allsync.biz/en_getfoldersize.htm
GetFoldersize allows you to determine the size of folders and sub-folders on your hard drive. Determine which programs are taking up your hard disk space. For each folder, you can display the total file size and the number of files and subfolders in the folder.
Here are a few simple ways to free up disk space
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Detect and Delete Duplicate Files
Uninstall Unneeded Programs
Defragment Your Computer
Do a Disk Clean-up
Make sure that there are no bad sectors or other errors on your hard drive by using the ScanDisk or chkdsk utility.
Transfer your personal data to an external Hard drive or Flash drive OR burn them on a CD / DVD.
Delete old restore points and keep only the the latest ones.
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