I have found that a lot of the mysterious buildup of data in Vista is from Restore Points and Shadow Copy files. These files are created automatically and can take up many gigabytes of hard drive space. They are good insurance in case of trouble and are worth keeping, so take some time to learn what they are and how to use them. Basically, they are a series of backup copies of your system files, [Restore Points], and your data files, [Shadow Copies]. If you are pressed for space, deleting the older Restore Points and Shadow Copies may be an acceptable compromise. The instructions below will help you some of the files you can live without.
Also, read this article about how to find and use your Shadow Copy data in Vista.
http://www.howtogeek.com/howto/windows-vista/recover-files-with-shadow-copies-on-any-version-of-windows-vista/ Disk Cleanup Procedure
Open the Disk Cleanup window: click Start> Control Panel> System & Maintenance> Performance Information and Tools. Then click on ‘Open Disk Cleanup’ in the left-hand column of the Performance Information and Tools window.
When asked to choose which files to clean up, select ‘Files from all users on this computer’. [User Account Control will ask you to confirm the action.] Then choose which drive to clean up [C: drive], and then wait for the Cleanup Wizard to calculate the quantity of excess files it can delete for you.
Vista will show you the Disk Cleanup window with a list of file groups that you can choose to delete. This will involve some personal choice on you part, but most of the file groups are small and not worth deleting to gain space. The hibernation files are big, but don’t delete them if you use hibernation. Click on the name of each group to read a detailed description. Check its checkbox to mark it for deletion. Click OK to perform the deletion.
This window has a very important second tab called ‘More Options’ [The ‘More Options’ tab does not appear if you chose to clean up only your own files on the computer.] The ‘More Options’ tab lets you remove unnecessary programs and lets you delete extra restore points and file Shadow Copies, which are like version backup files.
This disk cleanup tool will delete all but the most recent Restore Point and Shadow Copy files. Open Windows Explorer and see how much free space there is on C: drive before and after running Disk Cleanup to see how much space you gain.
All this does not necessarily account for all the space you are losing on your hard drive, but it solves part of the mystery.
