What OS?
I presume it is a Windows, did you patch your system?
Is there a Firewall in place for your system?
Are you on a shared network at home?
Are you using a Router as your gateway with SPI Firewall enabled?
1) After you format your com did you have a Firewall or enable your Firewall to block computer worm which propagated through network/TCP IP?
2) Did you install Anti-Virus Program after a Clean installation before accessing the Internet or connected to your network?
3) Did you patch your System OS after the above?
If the above sequence is not done upon a cleaning installation, not matter how many time u format your HDD and install, it is only a matter of time you will be infected with malware again and again.