There is no one product that can completely cover you.
You definitely need an antivirus and a firewall. If there is a trial version of a commercial antivirus on your computer, you can continue to use its protection or download a free antivirus. (Before installing a new antivirus, you need to uninstall the previous one; it is not recommended for you to have 2 antiviruses running.) You should also have an antispyware product.
Here is a list of freeware applications:http://www.bleepingcomputer.com/forums/topic3616.html
I personally use AntiVir (antivirus), AVG antispyware, Ad-Aware, Spybot Search and Destroy, SpywareBlaster and SpywareGuard.
Just be sure, whatever products you decide to use, that you update and scan with them at least once a week; it will ensure your protection.
You can also read this article:http://www.bleepingcomputer.com/forums/t/34773/preparation-guide-for-use-before-using-malware-removal-tools-and-requesting-help/
It's for before posting a HijackThis log (which I don't think you're going to do now), but it gives you steps to take for cleaning and what products are generally recommended here.
Just check out steps 4-8.
If you need any further info on any of the programs, don't hesitate to ask. Someone here will help you out.
EDIT: Took too long to type.
Edited by tink536, 17 May 2007 - 08:53 AM.