Dogscoff covered a lot of good things to do, but I'd like to add a few more to round it out a bit
-Wireless Networks-
You may want to consider changing the default IP address of the router. Linksys for example is 192.168.1.1 Dlink and others use similar ip addresses that are easy to discover. However, i recommend only messing with that if you know what you are doing.
-OS Security-
Dogscoff recommended a password on the BIOS and for bootup. Do this. Yes, several login screens are annoying, but the Internet is a bad neighborhood and you need extra locks on your doors and windows.
I'd also recommend password protecting the windows admin account (reachable via safemode) and disabling the guest account. XP home doesn't really disable it so you'll need to check out this.
http://www.petri.co.il/disable_the_g...windows_xp.htm
For firewalls, I recommend zonealarm. They have a free edition that's pretty damned good.
Dogscoff recommended changing the passwords after you secure the computer, I would do it at the same time I secure the OS but i'd do it this way.
1.) Setup a new master email address from a clean computer.
2.) Change the backup email address of all of her other accounts to that new master email, from a safe computer
3.) Now change all the passwords.
4.) Disable all forwarding email addresses and reply-to settings.
-Home Security-
Now, one more point of security. Change the locks on her doors and, if it is a rental or apartment or condo, make it clear to the property management and their staff that he is not to be allowed in AT ALL.