T400 WinXP Pro Product Key not working

[baertracks]

I recently purchased a used T400 (Model D474-DS2). The Windows label on the bottom of the laptop is intact with a readable product key, but the OS name is not visible on the label. I've seen that happen on a number of labels. According to Lenovo.com the OS for that laptop is WinXP Pro. So, I installed from disk Win XP Pro (SP1). However, when I try to enter the product key I receive an error message that "the product ID you entered is not valid". I have double and triple-checked the key, and am sure that I'm entering the correct code (and that I am not mixing up "8" with "B"s, etc.)

Has anyone else encountered this sort of problem? Is it possible that I must use a Win XP SP2 or SP3 installation disk for a T400? Are there any workarounds for me to complete the current installation and then apply the SP3 upgrade afterwards?

Any suggestions would be appreciated.
Thanks,
FRANK

[Neil]

Are you using an OEM XP Pro disc? Anything else, i.e., retail, VL, etc. will not work with the key on the laptop. Or, of course, use ThinkPad recovery discs, which won't require the entry of the key at all.