I'm not up on the free anti-virus software so have no opinion, but I'd just stick with the WinXP firewall. I haven't heard of any serious issues caused by any problems. Behind a router (which it sounds like will be the case), the software firewall is a backup anyway.

I do think that it's important to install only software you and at least one other person close to her are very familiar with. That will make it much easier for troubleshooting and training.