I use this nifty command every year or so:
Format c:
...I'm actually not kidding...the occasional "digital enema" does wonders for your system!
As far as software is concerned... I use the latest "Norton Internet Security" program from symantec. It does a great job of filtering out the crap and handles virus protection and bunch of other stuff all in one. They have a new one called Norton 360 that I'll probably buy next time my subscription runs out.
I used to use AdWare back when it was free, but then they started asking for money on some of the products. I haven't looked at it lately..I've been using Norton for the last few years..but it's a good (mostly) free product.
I'd prefer to pay for the all-in-one deal instead of having one program for firewall, another for antivirus, and another for ad removal.
Also keep an eye on your Internet Explorer Add-Ons if you are using IE:
Tools -> Internet Options -> Programs -> Manage Add-Ons
..That can fill up quickly with stuff you don't need anymore and I've had some items crash my browser on some sites.
I use both IE and Firefox...if I'm venturing out to new territory I'll use Firefox with everything turned off to keep trouble away.
--Fizz