well, limited account can install some programs, not all...you might be able to protect the settings that you don't want changed
let us know what settings are getting fujitztied, and maybe we can protect them
also, you might give him two accounts;
one account for installing programs as an administrator, but not to be used...and another account for use.
otherwise, you can just let him install his programs, and then change the rightes of the user to limited untill he needs to install more programs.
juggle this strategy around to suit your needs.
on a side note;
xp uses the pagefile inabundance when switching users...this is where the os strores the user information of the original user, when you switch.
it's very important for users with multiple user accounts to have a large pagefile, and certainly not a pagefile that is trimmed lower then the default.
just a side note, that's all