The memory is volatile - the contents are lost when the power goes off - the hard disk however tends to get filled up over time and you can delete programmes and files as you wish.
If you don't have the original disks then you can just clean out the stuff you don't need using Add/Delete programs and the Delete feature of Windows Explorer.
If you want a clean new-build of the operating system then you will ned to buy it from a store or on line - XP Home £60 Amazon
Peccavi, September 2009
