Spy-bot and Adaware. Both up to date, both used regularly.
Don't use internet explorer, get Firefox (
http://mozilla.org )
A competent Antivirus and firewall wouldn't hurt either.
Use common sense, stay away from dodgy sites. If these sites know a way to make money, they'll exploit it and you won't even know.
Oh, it might be worhtwhile noting the number that your ISP gives you, memorise the last few digits or whatever, just to make sure your pc doesn't dial a different one.
If you do use broadband, make sure the old dial up line isn't plugged into your old dial up modem, if you still have it.