MSE is available only for validated installations of Windows. When you
go to download it, you also have to pass their validation.
Apparently WGA thinks something significantly changed on your computer
that it no longer thinks that license is not in use on the same computer
as it was before. See what happens when you try to re-validate it. If
you run:
oobe/msoobe /a
then it will say it your license is [still] validated. If not, you'll
start the validation process which may complete with an online check but
may fail which requires you phone in to get a new product key.
http://support.microsoft.com/kb/916247
(You need to use IE for the diag page & it be running under admin
privileges.)