Try selecting some other text and paste it into eg Wordpad, then leave it for a while and try again.
If the URL pops up again after a few tries it sounds like you've definitely got some malware lurking.
You can visit
http://www.symantec.com and select their "Security check" feature (bottom left IIRC) which will scan your computer for viruses, trojans etc.
You should also get a program like Adaware or Spyware Blaster etc to scan for other rogue processes.