The only Things i can remember after a night of 23 beers + is that on xp for tweaking, best things to do is clean out your registry... on a mnthly basis. DO NOT EDIT IT IF YOU DO NOT KNOW WHAT YOU ARE DOING. You can just as easily use a program for that. Also, under run, type: msconfig
On the top you see tabs, the far right one shows startup or start-up.ini (because it goes for more then just xp). It'll show you a list then at the bottom half with check marks next to each item. Look threw each of those and any that you feel are not vital to run at ALL times; uncheck them.
Note: These Items all load on start-up. But also continue to run the entire time your on your computer slowing you down (real bad for gaming or system intensive programs) unless you use Cntrl+Alt+Del to remove them manually each time you start your computer. My suggestion is to remove all but stuff required. Xp will give an error msg when you uncheck something it needs to run right so your pretty safe.
The again under msconfig.... right next to start-up tab is a services tab. And again, uncheck all that you dont want or feel isnt required.
Other then that, under your internet options; make sure you dump your temporary internet files on a regular basis along with deleting your cookies. (located right next to temp files button. and then i also clear my history. Do this a a very regular basis. the more you install or delte, the more this needs to be done, but not exclusive to installing/uninstalling. I do it once every day or two.
From there, keep up with your defrag, anti-virus, and spyware removers. (I strongly suggest not having them run in the background the entire time. they are huge cpu boggers.
They arent "real" tweaks but its a good start on keeping it up to par. There is more but too complicated for a recovering drunk to go threw. hope this helps someone.
Cheers.