Part of your problem is that damn Celeron. No on-chip cache to speak of. You can install XP from a different CD but the CD key that came with that computer may or may not work. Nothing you can do but upgrade it to a Pentium.
Go to start, run. Type MSCONFIG. On the 'startup' tab, uncheck everything. Most people have an assload of programs running that they don't even know about. They start up because of a set of code in the registry that loads certain programs automatically. Uncheck em all and it will help. Go back afterward and select the ones you want.
Check out Spybot. It works. McAfee sucks a lot of resourse.
Finally, upgrade the ram if you can. Make sure both sticks are at least PC133. If you have a PC100 and a PC133, they will run at PC100.
HTH
Carl