I think you'll definitely need to speak to Microsoft. There's a number of changes of machine hardware I believe it will allow before you speak to them.
I'm surprised if you upgraded from win 98 to XP that OEM is legal as I thought OEM was for shipping with a new PC - there's debate going on in Vista forum about this.
Anyway, Microsoft contact details should be on their website. A quick search on google for "call microsoft activation" returned the following: