Please understand me here - don't misunderstand the purpose of this article. You absolutely must have up to date, real time virus protection on your computer. Even if you have no money to pay for one, that's OK, there are several excellent, free products. As far as a vendor recommendation, please feel free to peruse the neverending discussions in alt.comp.virus.
But even with up to date, real time protection (and please don't try running two installed virus protection products simultaneously), you will experience times when a second - and a third - opinion is necessary.
Fortunately, you don't have to download and install more software. In this case, there are several excellent free online services, that will scan your entire system, using applets running under the control of your browser, at your convenience.
Eric Howes Rogue/Suspect Anti-Spyware Products & Web Sites has another list of online virus scanning services, with more possibilities.
Not all of the above work with both Microsoft and Mozilla browsers, but choose carefully, and you should be able to find one or more that will work for you.