Here's a link to a few should be free
w w w.download.com/3120-20_4.html?qt=wipe+hard+drive&tag=srch&tg=dl-20
This is a great site to find free software and Kill Disk is listed there with a ++ listing so hatchet's advice in earlier post is probably a good one if you want to wipe the entire disk but there's also Stellar Wipe to delete selected files and internet history.
If you want to leave a working Windows install and your on XP then you could create a new user give (it admin status) from the control panel and delete all the old ones. You could also turn off System Restore, right click My Computer, go to System Restore Tab and untick the box - re-tick this once you've complete the actions below.
Log onto the new profile, then find & delete all of your sensitive files, check to see and if they are any files stored outside of your Windows My Documents.
Remember to check C:\Documents and Settings\ to see the users stores delete the old unused profiles , empty the recycle bin then run Stellar to compete the removal.
I Hope this helps
Keith.