Do you have a restore point from before your son downloaded the stuff?

There are also many free anti-virus programs you can install to help prevent such things from happening in the future. Here are three of the better ones that are out there...

http://www.avast.com/
http://www.grisoft.com/
http://www.kaspersky.com/

Why a password?

I suppose the password is to prevent the son (or others) from accessing the PC and downloading teh pr0nz. Only an ad-hoc solution at best. (EDIT: nevermind, the question has been answered lol)

Browsing using a restricted (non-admin) account, and a secure browser like Opera should be better. Add to that a properly configured router and good anti-virus solution like the poster listed above, and you have a pretty good defense against casual malware.

But nothing can save you if you intentionally tempt fate at shady pr0n/warez websites or download and run unknown .exe files.

EDIT:
Lighthouse, noted. My apologies for the digression.

To the OP:
Better download (from another PC), install and run kaspersky (there may be free trial versions) and see how bad the situation really is. Follow up with Spybot and Bitdefender scans.

If Vista is beyond salvage, better do a clean reinstall after backing up the necessary files to another device.

http://www.kaspersky.com/virusscanner is a free online scanner for a start. I don't think it actually fixes anything though.

If you want t go the free route just Make a few cd's with files and re-instal vista (Or run a system restore), Put your important files back on, and Ad a password for logging on and when your not using the computer, log off, this way the son cant get on. If He wants to, get on the computer, put in the password, and monitor him. You can also download a program called ActiveX which notifies you all websites that were visited in the past week or so.
Or If you want to pay, do the above, and get a program called NOD32. It catches virtually every virus and can quarantine them and sometime delete them. The program costs a mere 50 bucks and applies to all users and does an amazing job.

If the son is (edit by mod),then he's old enough to work and buy a good anti-virus program.I've used several programs and find Kaspersky on top of my list.Kaspersky may be a little pricey,but it does a solid job.

andrea,when yiu get your pc cleaned up go to control panel click on user accounts and family safety there you can get to the parental control.