Cannot Log on due to windows validation message

I'm new to this forum thing, but hope to find some much needed help.

I bought a used computer for work and went on microsoft.com to make sure
that the Windows XP Pro already installed on the computer was a valid version
because my employer said a bootleg copy wouldn't work with their system. I
used the validation tool on microsoft's website and it said it was valid.

Here's the problem: I would get a message asking if I wanted to validate
windows because it would need to be validated within 6 days (paraphrasing).
I clicked yes and it said "windows has already been validated". "Click OK to
close". I would click OK and my comp would work just fine. The next day,
same thing, but only 5 days left. The countdown continued until today.
Instead, I got the usual request asking me if I wanted to validate, but
whether I click Yes or No, my comp shuts down. I can't log in or do
anything. I've put it in Safe Mode and gone through the exact same message
screens as described above, resulting in my computer shutting down.

Help, please (begging here)

Thanks
--
Kristen