Virus Problem in windows XP???

Hi,
I am having huge troubles to use my PC. Why Operating system Is windows XP professional service pack 2 in this version i am facing too much Virus problem.
The system become too much vulnerable when i connect to my LAN connection or to my internet connection. I have to format my PC in every month twice.
Its really frustrating and time consuming. So want to get rid of it. Anyone has any Idea?
My LAN connection is with large people more that 80 people is connected all time to my LAN connection and its very necessary to stay connected so there is no alternatives i must have to connect to LAN.
Well i will be glad if any one tell me what to do its very important to me and there is no other Operating system available to me. So what can i do please tell me please.
