Anyone can fall victim to viruses and Trojans. Prudent behavior may reduce the risk. The risk itself, however, can not be eliminated. If you're unlucky, all you need is a web visit to become a Trojan horse (driveby attack) victime.
So it's very important to improve wallet safety as discussed by Gavin for example.
But as long as we have to use insecure Bitcoin software (Bitcoin 0.3.23-win32-setup.exe) we should be arare that any Trojan can steal all our coins easily if we used default install procedure of Bitcoin.
One possible way to improve wallet safety might be to install the Bitcoin software within a virtual machine (VirtualBox for example). What is the opinion of the forums about it?
Another problem arises out of the fact that your own faults (crash the virtual machine, problems with backup and restore of wallet.dat etc) can easily lead to a loss of all coins.
It seems therefore desirable to integrate a a foolproof backup / restore function into Bitcoin software.