After about 3 months, I am finally re-jigging the old 'puter.
I've taken it back to an early clean state and removed the crud which was already there - Real Player and AOL and the like.
As it happens, the XP installation already had SP2 installed, so that saved a bit of messing around.

I ran Hijack This before starting and will run it at intervals when I start making bigger changes. I am backing up my changes incrementally using Acronis True Image, so I can return to any specific point in the process and re-start from there if I have a problem.
Belarc Advisor is definitely useful.
Win Patrol is alerting me when programmes try to change the registry, so I can okay or deny the changes.
One useful trick with Win Patrol is it's cookie management. It makes keeping selected cookies (such as Mudcat) easy and then allowing you to delete ones not on your list to save.
Another useful one is the ability to set a blacklist of cookies by typing in some text from the cookie name. It will then automatically delete any cookies containing that string. I have been using it in conjunction with Ad-Aware and Spybot S&D. If they find tracking cookies on my machine, I can use WinPatrol to prevent them being put onto my computer again.

Currently I am downloading and installing Microsoft Security Patches.
Would you believe that from a total of 76 patches, all except 15 were security flaws which could allow an attacker to take over your computer.
        The remaining 15 mostly just stop the computer working at all.

What a wonderful computer system...

Quack!
GtD.