Trust no exe + Firefox = reboot?

I decided to try "trust no exe" on the kids' XP Home computer in combination with LUA and SuRun to implement a fake SRP (rather than trying to hack gpedit.msc into XP Home or hacking XP Home into fake XP Pro). At first glance things seemed ok, but then when I tried launching Firefox the system rebooted. There is a post on the mozillazine forums describing the same problem but unfortunately there are no solutions posted other than granting access to "Documents and Settings", which kind of defeats the purpose of trust no exe... Has anyone else encountered this problem and have a fix? I am reluctant to install HIPS on this system because it is used mostly by non-technical users and I don't want to hear screams of "Dad, the computer's not working!" all the time...