If having problems logging in as administrator to do a restore try booting
in safe mode....
"A description of the Safe Mode Boot options in Windows XP"
http://support.microsoft.com/kb/315222
If that's not possible because of password problems try..
"How to log on to Windows XP if you forget your password or your password
expires"
http://support.microsoft.com/?kbid=321305
or possibly the Emergency Boot CD on this page (but I've never used it)..
http://www.petri.co.il/forgot_administrator_password.htm#3
You may need access to another PC to download and make this CD but it says
it allow you to boot and then change user/admin passwords.
Once you can log in as administrator you should be able to do a system
restore back to before you got this problem.
PS: If you aren't running antivirus S/W (and/or you don't have the firewall
enabled) then best physically disconnect your PC from the internet before
you start the above and don't reconnect your PC until you do).