Hang on in there!
I had to reinstall XP yesterday and only went to activate it today. It wouldn't work - not even a failure message (volume license) - under IE or the Firefox extension.
After a spot of searching, it turns out there have been problems with WGA in the past week
One solution I found involved setting the clock back to 6 July. I did this, installed more (optional) updates and the WGA icon is the tray disappeared, along with the activation reminder. So that, er, "fix", seems to work.
Best of luck!