that depends upon the no of services you use on your system. for e.g. let's say you don't have internet connection at your home. then you might not need any messenger & so the reduction of no of processes. likewise there are many processes.
Well, I had formated my pc many time, and i noticed that a fresh installed pc, with no other softwares, XP SP2 has 18-20 process running...
Now with the antivirus, I hv AVG, i hv 3-4 more, than 3-4 other and making a total of around 26-30 process normally, just add the softwares or programes that you run extra.. like this browser and msngrs....