If you are bonkers paranoid....click here but all you need to do is reformat it unless you are a spook from the CIA...and then you wouldn't be asking this.
For XP....Pop your Windows XP disk in the CD ROM drive and wait for the installation menu to come up. Choose "Install Windows XP" then it will go into a new screen that lists a bunch of steps. From the from the pulldown menu choose "New Installation (Advanced)" and let the software do the rest.
It will wipe out all the data on your hard drive and put a fresh copy of Windows XP on the system.
For 98 etc......click here