If you've ever run the Windows Disk Cleanup utility, you probably discovered that your temporary files occupy a significant amount of space. You might select the Temporary Files check box in order to allow the Disk Cleanup utility to delete the files in the Temp folder, but the Disk Cleanup utility will not remove all of the files.
The reason for this oddity is that the configuration for the Disk Cleanup utility does not allow deletion of files accessed in the last seven days.
By altering the LastAccess value in the registry, you can configure the Disk Cleanup utility to delete all the files in the Temp folder regardless of the last accessed date. Here's how: Open regedit; Go to HKEY_LOCAL_MACHINE\ SOFTWARE\ Microsoft\ Windows\ CurrentVersion\ Explorer\ VolumeCaches\ Temporary Files. Locate and double-click the LastAccess value. When you see the Edit DWORD Value dialog box, change the Value Data setting from 7 to 0 and click OK. Restart Windows.
This works on XP and Vista too.
In Vista, there is also an Enhanced version of the Disk Cleanup tool ! It basically just has more options for you to choose so you can delete some addl files.
To access it just run it or type cleanmgr /sageset:1 and hit enter