Try and activate it online this time without calling Microsoft.
If there are no hardware changes then it should activate without a problem.
If it asks for a credit card number, then it is the trojan. Do not continue.
You can attempt to get rid of it, but I say why not do a clean install of XP? It 
has been four years and that's allot of time for XP to get bloated with junk.
Besides, after doing a clean install of XP, you will find the PC allot more responsive 
(aka quicker).
Be sure to transfer any data you need to an external hard drive, burn to DVD/CD, 
or however you prefer.
Just a thought.

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