What browser do you use (IE6, IE7, IE8beta, Firefox, ...) and what version of XP (Home, Pro, MCE, what service pack ?)
When did the problem start ?
If this is IE 7 try to reset the browser's default settings :
This will erase your history (no big deal) but it'll also erase the password list if you had IE remember the passwords for you. Make sure you remember all the logins and passwords you use online before you reset the settings.
If the above didn't help install firefox and see if it works properly :