I'm wondering if you might have a bug in your computer. I do not have XP to be able to recreate your problem. As a suggestion, PCHF has a good Security group of people who can, if nothing else, eliminate a bug as your problem.

If you're interested, this is where I'd go to be sure.
http://www.pchelpforum.com/xf/forums/am-i-infected.479/
Reading the first two threads in the forum may help you decide if you want to post there.

Good Luck

P. S. - There is no cost in giving it a try. They are volunteers there, just like here.