What we're tryin' t say, daddymurphy, is a competent antivirus and firewall setup, used properly, oughtta do that just fine. If you suspect you have a current infection, tell us why - what makes you think so - and what, if anything, your antivirus says about it.
Even a good, and properly configured and deployed, antivirus can miss some things - while you want only one runnin' full time on your machine - your primary antivirus - its not a bad idea to get second opinions on a fairly regular basis, which ican be done a couple different ways; you can install and keep updated antoher vendor's antivirus, but configure it as scan-on-demand only.
Even more than ons additional is no problem, as long as only one is configured for real-time protection. There also are the online or downloadable free scans. You can even go to the extent of havin' your primary AV, a back-up or two or more, and in addition use the online and and/or downloadable scans - rememberin', of course, to have only one primary, real-time app at any one time, and to disable your primary before employin' any of the alternates.
In the event of a significant new threat, the major vendors, such as McAfee, Symantec/Norton, Trend/PC-Cillin, AVG, Kaspersky, Panda, and many others are quick to make available downloadable stand-alone cleanin' tools. NAI/AVERT's free "Stinger" is particularly valuable in that regard; frequently updated, essentially it targets just the latest-greatest threats of whatever sort.
Microsoft's own web-based (also downloadable)Malicious Software Removal Tool is pretty good for users of recent versions of Windows, and Microsoft's AntiSpyware (Beta}, also for users of current flavors of Windows, is fairly adept at catchin' and dealin' with the sorts of trojans and "trojan-like" ploys favored by yuckware pushers.
If you insist, a reasonably functional free-to-try-basic, nag-to-pay-to-get-upgrade "fully featured" product that is aimed specifically at trojans and worms is
SwatIt. It does pretty much what it claims it does, and comes with no undesireable payload of its own.
Be aware that the stuff out there on the 'net that is particularly obnoxious about advertisin' - the most eye-catchin', enticin', urgent-soundin', compellin' sorta stuff that visually yells "You might be at risk! Scan now for
free!!!!!" most likely is specifically what you are
NOT lookin' for.
And no matter what is installed on a machine, and no matter how it is configured or deployed, the most important link in the chain - all too frequently the weakest link in the chain - sits on the other side of the keyboard from the monitor.