[Ian Robinson]

Ditch Norton AV (uninstall it completely) after your have done a system recovery (pick a date (restore point) that is well before your computer got really slow BTW).

Get a copy of SPYBOT and run it , then uninstall it. (FREEWARE)

Get a copy of LAVASOFT ADAWARE (FREEWARE) and run it (regularly), this will get rid of malware that has been installed in the background when you visit websites , that are not viruses , this will get rid of garbage that Spybot and what ever the new antivirus app your decide to use missed, but which still use lots of resources and can really slow the computer down.

Reinstall XP and the updates (up to sp2) if all the above doesn't help. I've not bothered downloading or installing sp3 - I've heard bad things about it.

If you've got lots of shortcuts to stuff on your desktop - delete most of the shortcuts - they will consume resources even when not being used or viewed.
I've only got 9 shortcuts to folders or apps that I used frequently + the Office Slide Bar, and am often amazed at how many shortcuts people have in their profile .... seen some people with a screen full of shortcuts (who wonder why their computer takes forever to startup .... been there and done that myself).

Tidy up / purge the program list - I use a rule of thumb that if I haven't used a program for 12 months or more - it's unstalled.
Tidy up / purge the shortcuts directory .... easy to accumulate lots of bookmarked shortcuts to urls that just clutter things up - if you aren't sure - copy all your shortcuts to a floppy or CD , then be ruthless in purging.

I have updates active , but only to advise me there are updates , which I manually install.

GET MORE RAM - this is the very best and cheapest way of speeding up an opsys and a computer, RAM is pretty cheap , and a lot cheaper than replacing a computer. The more RAM you have , the less often the HDD has to called, HDDs are the main limiting factor in computer speed and really have not become all that faster (seek time and platter rpm) than they were several years ago , they just got bigger.

Get more RAM.

This is what I would do.

This is what we learnt in ELEC4700 (Advanced Computer Engineering) a few years ago when I did that subject as part of my second engineering degree.

PS : there are some good Registry Repairers about that are FREEWARE , get one and run it , that can't hirt and may even help.