Call Of Duty lag

I have been playing call of duty for awhile. After I reinstalled my Windows XP, it seemed to kick me out of PB servers more often for losing key packets. When it kicks me out for losing key packets, the game freezes and, I literally have to restart the whole computer by hitting the power button. It did not used to do this before I had reinstalled XP. After I have restarted my computer and run a ad-aware scan, i rejoin a PB server and it seems to be real laggy. I have tried everything i can think of to fix it, but it does not work. If anyone could help me, I would appreciate it!