Clear all data and start fresh erase all...

What OS are you using?

In the OLDEN days this was a good idea, but not necessarily any more....

Before you do anything, you must create recovery disks, if you do not have them, and create user backup, especially focusing on accounting files....

Then you insert Windows disk and perform fresh install, this wipes everything, still this is not needed as often as people seem to think...

Doc

If you have the windows instalation cd then your in luck if not then your going to have to go out and buy them, once you have the cd boot off the cd and follow the instructions, you'll want to to do a clean install and that means that your going to have to use the format option after that just let the cd do its job and note this will take a while depending on your system and how big your hard drive is

FORMAT THE COMPUTER USING WINDOW XP OR OTHER OS SOFTWARE AND AFTER FORMATTING AGAIN LOAD THE NEW OS.