First thing to ask is do you have any sort of firewall running? If you don't have at least the XP firewall running when you install XP, it can be compromised within minutes of being connected. Much faster than you could ever hope to download and install all the necessary security updates.
Another question that has to be asked, is there ANYTHING about what you're doing that might bring this about? Using any pirated software, including the operating system itself, visiting any questionable sites (porn, hacker, warez), anything like that? How about file sharing programs such as Kazaa, LimeWire, DC/DC++? You can lie, but then it's all the harder for us to help you.
Finally, do you have any kind of virus protection installed? If yes, is it up to date?
I am running Windows XP on a Gateway GM5084 Media Center, and for over eight months, I've been having to deal with what I believe are hackers. I have reinstalled my system, both destructive and Partial, a number of times and still haven't got rid of them. Yesterday (11/19/07), after my last reload, my username and Administrator access rights were suddenly changed from the Owner/Administrator to just someone who has limited access. I don't know what my user name is because there isn't a Welcome screen.
I haven't been able to find them under "Users" in the Control Panal. Unfortunately, the Control Panal itself disappeared as of last night. The closest I've come to Identify them is when I ran a quick scan as soon as I knew something was wrong, whereby the name BUILDIN/ADMINISTRATORS appeared.
If anyone has experience with this or has a new suggestion, I'd be all ears.