If you don't want spyware, another very effective measure is to run a Limited / Restricted-User account. These don't have the privilege level to install software. Firefox is not invulnerable to attack either... we saw some Dialer exploits at work against Firefox (stopped by our antivirus software, of course). I think that the first thing to do, in any case, is to beware what you install. If you download and install something that contains spyware, you need to break yourself of that habit. 😉
I was helping a co-worker clean BargainBuddy off his home system, and used Spybot Search & Destroy 1.3, AdAware SE 1.05, and eventually a 15-day trialware of Norton Antivirus 2005. The surprise is that Norton 2005 found about 20 items that the anti-spyware apps missed 😕 So if you're up for some new antivirus software, you might grab a 15-day trial of Norton and see if you like it. It does slow the system down, but it seems to have significant anti-spyware capabilities.