How well does your anti-virus program protect you? It's hard to say for sure, and many test results available on the internet and printed in computer magazines fail to represent the real threat or the genuine effectiveness of desktop security products. Usually they default to saying something along the lines of, "most anti-virus programs work pretty well, but product x has the best interface while product y is the cheapest."
Interestingly a new test by German magazine c't claims that many popular anti-virus programs have become less effective over a year. Although details of the test are not generally available, it's clear that c't tested the ability of these programs to detect 'unknown' threats. Providing effective heuristic abilities has always been the holy grail for security companies, because protection based on recognising known threats is always going to provide attackers with a window of opportunity.
Sadly for those who cannot read German, "the full test is only available in German at the moment, in print form, in c't 1/08."