If your system is compromised, it doesn't matter what IP address you have (dynamic or not). Malware on your computer can "phone home" by itself. Also dynamic IP addresses from ISPs do not change as often as people think - if you are always-on/connected, you can hold the same IP for quite a long time (days, weeks, even months).
If you were to completely wipe the PC and start from scratch then it can wipe out an infection, however you could also try cleaning your PC.
Take a look at our
HijackThis 5 Step Process and post a HijackThis log in our HijackThis Log Help board, where a member of our security team will assist you in cleaning your PC. Our security team is very busy, so please expect some delay in reply.