I think you are ok unless you are using "fast user switching".....with fast user switching it is keeping BOTH users accounts open etc.
I think if you log out of the one and then log onto the other one you wont really be using extra resources.
run "services.msc" from the run box...then youll see "fast user switching"....change it to "disabled".
while you are at it you could check out
www.blackviper.com and check out his 'XP services configurations'...he lists all the services that are running by default and which are totally unneeded...so you can turn those off to save resources....it does help give a little more speed to xp. I use his "safe" config except for a few changes.
typical stuff you DONT need running
alerter
indexing service
wireless zero config
portablemedia serial number
smart card
smart card helper
and a whole long list of others that can be disabled to free up resources.
JP