The only 100% effective solution is don't go on the Internet with XP.
Look into a 'dual boot' arrangement, with some flavor of Linux for those on-line tasks.
I recommend Ubuntu for folks with limited know-how about other operating systems, and don't have the time or inclination to learn an entire operating system.
It's point and click; can be arranged to resemble Windows pretty closely; and the time you save not having to update, scan, download fixes, re-update, re-scan repeatedly can be used to explore this dandy, free, and flexible operating system.
See how easy it is: http://www.psychocats.net/ubuntu/installing
There are other flavors, and set ups, but this installs in about an hour ready to go to the 'Net. Experiment with others as you have time.