Norton Security/Virus Scan:
http://security.symantec.com/sscv6/WelcomePage.asp

The above link provides a point where you can perform a security or virus scan of your computer over the internet, with Norton's up-to-date security definitions. You can also download a program that can automate the scanning once per week. All of which are free.

From my understanding, this is not an active security application, it does not monitor your system activity, so to solely use this product without virus protection and spyware removal software would not be wise.

Also AVG provides the same solution, yet is more so an active living breathing application that monitors your system, as opposed to just finding viruses after the fact. I've heard mediocre results with this program. Anyhow, here it is:

http://free.avg.com/

I'm sure there'll be plenty of more web-based (like the Norton described) that will be appearing. I'm surprised that Firefox hasn't integrated spyware/adware monitoring via a built-in module.

Keep in mind that [see previous post] that you can save your computer from malware with Microsoft's MSRT and Norman's Malware Removal tool, all free.