i don't think even advanced users should disable system restore, you should temporarily disable it when you have any malware on your system because the malware can be in your restore points, once the system is clean turn sys restore back on & make a restore point, will be handy if you ever need to restore your pc to an earlier time.
"It's got a fresh xp on it... for 1 day. How on earth do I have a trojan?"
is windows updated? sp2 installed? browser? firewall? nature of online activity?
If you connect to the internet without an old version of xp without a firewall and an ancient version of IE, you will get trojans & spyware within minutes.