You can get an ISO of Win7 herehttp://www.w7forums.com/official-windows-7-sp1-iso-image-downloads-t12325.html
Now if you go this path, you can erase(format the drive.) A quick format is technically not an erase, but just tells the computer the drive has not data and over time it gets overwritten. If you do a FULL format, it takes forever, but will set the full drive to ZEROs so no data should be left(unless you are dealing with someone REAL good).
You can also get utilities to fill the drive with random junk thus overwriting everything.
I do not think a virus will go to the point of looking for recoverable data and key loggers only log while you are using the system.
I would just quick format that sucker with the Windows DVD. Once you get it installed you need to install drivers then activate Windows.
A fresh install has NOTHING but windows, so software and drivers are gone as well.
If the new users have issues with messing up systems, I would make a backup image(ghost/easeus/ect) of the system just before they get it.
It would also not hurt to create a second partition(third if you count the system reserved) to store files on so that if Windows gets messed, at least you can just restore everything on c : without having to worry about files.
Its good to see a system getting a second home vs being trashed