Hey Frank... welcome to the forum.
The only thing I can think of is that it's a DRM (digital rights management) issue within WMP 11.... As I use WMP very little, let me see if I can walk ya through this: (btw, i'm sure there will be better advice to follow from others...) The problem as I see it is this: If you didn't get into the guts of WMP 11 when you copied the music, it was probably saddled with the Microsoft DRM default handle. So when you went to play it on yer other computer, It kicked in (i think it's a copyright thing) and wouldn't let ya do that. If , in the future you want to stop that, then you gottta acsess the 'Options' menu pull down under 'Library' allthough it is available through different ways... Lets just stick to this point:
Under 'Options' select the 'Privacy' tab, and Un-Tick everything.
The key here is the one 'Tick' in "Download Usage Rights Automagically when I Play or Sync a File"
I'm fairly sure that's where the problem lies.... Again: I don't use WMP much, so I'm not 100% on this... but give it a try.
BTW, you will have to re-do the files on yer HD, if you want to transfer the files that way I believe.... The other possibilities are: Copy and paste the files from the HD to the Vista notebook, or Burn baby burn.... {;>