Laptop cd key question.

Hello,
I bought Windows 7 Ultimate edition and installed it on my laptop. This laptop has a Windows media centre edition 2002 cd key sticker attached.

I then went on to install vmware and tried to install windows xp mce on it.
Now I lost the recovery disk supplied and the laptop manufacturer asked me to pay in order to re-deliver the disk. So I downloaded Windows XP MCE 2005 OEM edition instead but it does not take the cd key I was supplied it with.

Here is my question,
a. Why isn't the windows accepting my cd key prior to the installation? I thought the cd key supplied from laptop are all OEM copies and I am using the same machine?
b.Could it be due to the different cpu and ram amount allocation? [i.e. allocating 1 cpu to vmware while the machine has 2?] I upgraded my hard disk and installed additional 1gb ram. Apart from these all other devices are identical.
c. Could it be because I am using MCE 2002 cd key to MCE 2005 copy?

Many thanks in advance.

DMHolt57 Thank you :) I thought they were more a less same copy. I will try with the other version.