how to identify a worm attack?

I have a Windows XP on a P4 2.6 with 256Mb RAM, but the performance is not good, I have scanned it with most of the anti-spyware prog. and the network connections runs well for first few kb of packets, then it goes seriously slow till all network resource couldn’t be access, same for internet connection.
I have disabled all the firewalls, so is this a worm’s doing? how to get rid of it? thank you.