Is Your PC a Zombie?
If you've been diligent, you've installed a firewall and anti-virus programs on your computer. You should also be running a spyware and adware removal tool regularly. If you don't have any of these, be sure to check out my recommended list of free firewall, free spyware removal, and free anti-virus tools.
So what's the next step to ensure that your computer hasn't somehow been taken over by some program? Check your IP over at MiraPoint Check IP. This free service will compare your IP to their database to see if your IP has been flagged as one that sends spam without your knowledge i.e. it is a zombie PC.
Two other useful programs are Active Ports and TCPView. Both utilities show you a list of programs running on your computer and their current network connection status. If a program is listed and you don't know what it is, you should look in to it. Also, if a program is listed that you do recognize, but that you aren't currently running, then you should look in to that as well in case a malicious program is piggy-backing off of another trusted application.