There are many ways to remove XP Antivirus, both manually and through programs. After being infected with this myself, I had the exact same problem as you....XP Antivirus was blocking my access to any and all websites that mentioned anything about XP Antivirus 2008.
What you need to do is download Malwarebytes Anti-Malware 1.28 from here
Save it to your PC and then install. At the beginning of the install, make sure you have 'Launch Program' and 'Update Program' boxes ticked. Once installed, perform a 'quick scan' and then follow the directions. If the program asks you to restart your computer, then please do so.
If XP Antivirus blocks access to the page I linked above, then go to an unaffected computer, download the program, and then burn it to a cd and install manually on the infected pc.
This was the easiest way I found for removal and it completely removed all traces of XP Antivirus. After my PC was disinfected, I couldn't believe the performance gains I got back....XP Antivirus is the worst of the worst.
*Edited to include that preliminary use of this program is free...as I used it last night to disinfect my machine. As it is 'shareware' I'm not sure when it 'locks' and starts prompting you to buy it, but it won't cost you anything if you want to use it to remove XP Antivirus.
Edited by snapgiggle, 17 September 2008 - 10:12 AM.