Pull one of the infected drives off and attach
it to a good PC as drive D or such, manually
wipe out everything in the Temp folders under:
C:\Documents and Settings\username\Local Settings\Temp
Also do the same for the entire Temporary Internet Files
folder (You should keep the Temp folder but you can
just delete the entire Temporary Internet Files folder.)

Then run a sweep with a non-Norton, non-McAfee
antivirus (F-prot, Kapersky, Nod32, F-Secure, even
the AVG and AVAST! freebies are better) and
with Spybot Search & Destroy 1.4 and Ad-Aware.

Do a full update prior to the sweep.

Put the hard drive back and see what happens. If
some Windows files were deleted and you have an
XP (I'm assuming) Installation CD, you can try an
in-place "repair install" as described he
http://www.michaelstevenstech.com/XPrepairinstall.htm

As far as protection goes, your kid's PC should
have no access to anything on your PC's. Restart
in Safe mode and logon as Administrator, and then
change its password (it's blank, believe it or not, on
XP by default.) Install a good firewall like Kerio or
Sygate, not so much to detect break-in's but to
flag suspicious programs trying to access the
Internet or networked PC's. Microsoft's AntiSpyware
Beta actually isn't bad, but you might be put off by
the new "Genuine Advantage" validation crap.

Good luck.

-BC