There are many ways that she could be logging on to your computer, the most likely, since it is built into Windows XP Professional is Remote Desktop.
You can disable Remote Desktop by RIGHT clicking MY COMPUTER (either in your Start Menu or on your Desktop).
Click the REMOTE tab, and see if the ALLOW USERS TO CONNECT REMOTELY TO THIS COMPUTER is checked. By default this setting is turned OFF, so if it checked, then this is most likely how she has been accessing your computer.
If it wasn't checked, then she is probably using one of many 3rd party software programs, and it's next to impossible to guess which one. I may be able to help one of two ways, Either download and install Hijack This: http://www.majorgeeks.com/download3155.html
scan your system and save the log. For security reasons, I recommend that you go to my website, and send the log file to my email, so I can try to determine what program she is using.
Go to my website, contact me, and we can make arrangements to perform a remote assistance, and I can bring up your MSCONFIG and see what programs you have in the startup to determine how she is accessing your computer.