I have problem with my OS (Windows Vista Premium 32 bit). If I enter Vista, its asking for a product key but I don't know my product key.
Please note, I have dual OS in my desktop.
If the OS already was installed when purchased then there should be a sticker on the box (on the computer).
If you've got your own DVD then it will have come in the DVD case.
If you need to get a serial back and don't have physial access then you can try to retrieve it direct from the computer it was installed on using magical jelly bean key finder
The Magical Jelly Bean Keyfinder is a freeware utility that retrieves your Product Key (cd key) used to install windows from your registry. It also has a community-updated configuration file that retrieves product keys for many other applications. Another feature is the ability to retrieve product keys from unbootable Windows installations.