Installing SP1 to Dell OEM version of WP11

[Bill]

I am trying to install SP1 for WP 11, and after it downloads it won't
install and gives me a message saying that "the upgrade patch cannot
be installed by the Windows installer service because the program to be
upgraded may be missing or the upgrade patch may update a different
version of the program"

I have tried installing SP2 just in case I already installed SP1 (even
though the version numbers indicated that SP1 was the correct one, and
I got the same message. My OS is XP Pro, and I have disabled Norton
Internet Security during the install and download process.

Any suggestions on how to fix this would be appreciated!

Thanks, Bill