I dont know XP Pro x64 but its sounding more like a trojan, Have you swept your system for problems Spybot search n Destroy is a good free program
ALSO Make a new thread (LimeWire/ XP Pro x64 Compatable ? Help.)
Check your system using any of these programs (I reckon using all of them):
http://www.lavasoftusa.com/software/adaware/(free)
http://www.grisoft.com(good free AVG)
http://www.ewido.com(>>now called AVG Anti Spyware<< free 30 day trial -the best in my opinion)
http://www.ccleaner.com/download/(free)
Spybot - Search & Destroy (FREE)
If you can't get rid of it go here and get some free computer help:
http://norbiesworld.co.uk/computerhelp
You can also read here point 4 in the post of LOTR....:
http://www.gnutellaforums.com/showthread.php?t=41432