Better yet, download the Ultimate Boot CD, fire it up, get the latest virus scanner upgrades to the scanner(s) it has and clean your PC from that CD.
Not the Ultimate Boot CD for Windows, since you have to add Windows files to it when you create it, you'd possibly wind up with an infected CD - but the Linux based ultimate boot cd in iso form should be burnable and not become infected.
You should probably have a software firewall downloaded onto a CD (get one from a friends computer that isn't infected) and then reinstall your OS again, while disconnected from the Internet! Then put in that firewall cd (zonealarm for instance) and install that BEFORE connecting to the Internet. That way direct attacks will be harder even though XP is vulnerable out of the box to all kinds of directed assaults.
Then go directly to Windows Update and download every security patch in sight before installing an anti-virus scanner...
