@presha
A targeted virus, one that just opens wallet.dat and sends it off, one that you ran on your own accord, will not be detected by antivirus. No antivirus company yet looks for programs trying to access your wallet.dat.
Please tell us what you have downloaded and run lately so that we can find the program doing this.
I'm 99% sure now that someone hacked into my dropbox account, where I stored the first backup of my wallet.dat a few months ago, when I started mining. The file was compressed and with a rather long password, but I assume It wasn't a problem for some bruteforce app to crack it.
I'm so glad that this was my very old backup and I lost only 0.46 BTC (doing encrypted backups only on usb drives now)
btw
http://techcrunch.com/2011/06/20/dropbox-security-bug-made-passwords-optional-for-four-hours/goodbye dropbox...