If you manage to keep the content of drive C small, you can create a drive image once your Windows is working OK and save the image in another drive or DVD.

When your Windows get crazy,  infected by a virus or installed something that screwed Window you don't need to re-install Windows, format the disk, install all the drivers and apps again, you only need to restore the saved image booting from a CD and in 5 minutes your latest stable Windows is back again.

You can use TodoBackup from Easeus to create the images and is free !

The other problem with Windows 7 is the folder Document and Settings that has the access forbiden and you even can't know what is inside. All the trash is inside this folder and this folder grows for ever. I don't know how to make this folder have the access open to clean all the trash inside.

Stupidity also evolves!