I suggest that you download and run Windirstat. It does a lovely job of showing you where space is used.
As to reverting the files back to the first install, you need either an installation disk or the manufacturer's reset process. Some makers put a hidden partition on the machine with an image for re-imaging the machine.
The disadvantage of that is that you have to install years worth of updates and all your software all over again.
By default, XP stores information to uninstall every patch that is installed. This can add up to a large amount of storage space. There are ways, both manual and automated, to scrub some of this stuff. This will eliminate your ability to rollback any of these changes. Look for folders with names like $NtUninstallKB939683$