Even re-installing
Windows, could possibly leave certain data behind.
What you need, is a simple, free file shredding/space-wiping application.
This will definitely, purge all of the
cluster-chains, the areas on the HD where the files are stored.
When you delete a file within
Windows, it will usually only delete the file header, leaving the main body/data of the file intact,
residing in the
cluster-chains, which could possibly be recovered, by using file recovery applications.
There are many free file shredding applications available, just Google for file shredders.
There is a litte application called
BCWipe.
They describe using it, is a 'Military Grade' procedure.
It's a file shredder, but it also wipes drives/free space/swap file etc, as well, so for example,
you could wipe the free space on the drive, after you 'delete with wiping', the files.
The more passes, the less chance of it being recovered, bearing in mind , the old DoD standard is the 7 pass character rotation wiping,
and this app goes up to 100 passes, or maybe more, which would not really be necessary though imo, and would really take ages.
Download here:
http://www.jetico.com/wiping-bcwipe/
A 30 day trial though, lucky I got my version, full, ages ago.
There is also another free application here:
http://www.fileshredder.org/
There are very many more, if you Google for them, you will see them.
Hope it helps.