Well every download (never mind P2P) starts at high speeds(especialy from servers) but it's constantly drops till reaches your actual download speed. Anyways if you are suspecting that your speed is to low you should conntact your ISP and ask them what's happening with line is there are any problems and they should be able to tell you if anything is wrong, as I said my opinion is that your problem is not XP related.
One more thing you don't have download limit do you???
