You have have a bit more knowledge on what he is running and how powerful his system is.
There are always services and programs running in the background, you just have to know what you have.
Diskeeper running on full auto will degrade system performance as it defragments the system during idle.
Norton likes to scan inconsiderately.
Most people upgrade to symantec enterprise.
In newer XP version, you do not need an external firewall, windows does a pretty good job of defending itself using its internal firewall, especially if your behind a router.
There may be other software such as automatic update doing some useless junk in the background.
Check task manager for the processes that are heavy in processing and heavy in memory consumption. If you dont need them, or are one of the processes i mentioned, you can disable them in either
services.msc
or
msconfig
Not all the services in msconfig are the same as in serives.msc.
Dont disable anything you will need, especially plug-n-play.