How do I transfer an iTunes library to another account on Vista?

You should be able to go to the Library’s location on the disk (most likely in “My Music” and copy that iTunes Music folder to either a shared folder or an external drive, then go to the new account and iTunes—> File—> Add to Library…

you cannot play music on different account, you can authorize five computers under your account.

That only applies to iTunes Store-purchased DRMed music and yes, you can authorize that account to play the music.

how come, I didn’t know; tell me, please