when you start outlook something complains that your version of XP Pro is not legitimate? What is the exact wording? It's possible that this is actually not a popup from MS/Office at all, but some form of Macro/VBScript outlook virus. (do you get the same popup with any other office application?)
The site MS redirected you too was probably one of their Anti-piracy sites. Standard procedure, I think.
But the thing is- I can see outlook- or another office application (does this occur with other office programs?) complaining about an illegitimate Office installation- but I really don't see them complaining about the OS itself. This makes me wonder if, in fact, this message is being generated by some sort of infection. (this tactic,. (claiming the OS or application is illegitimate) is commonly used to trick people into buying a program or something similar (or maybe just to cause trouble for MS support staff).
So, let's get some details then:
What version of Office/Outlook? (2000,XP,2003,2007...)
What XP and Office service packs?
Can you get the exact wording of this message?