If your Serial Key or Product Key of Windows Vista is blacklisted by Microsoft.com & you are unable to update your PC with some major updates, Then Sure you have to purchase New Copy of Vista from Microsoft. After Purchasing the New Vista DVD you will get a Legal Serial Key or Product Key.

If you want to Use that Serial Key or Product Key in your current Vista OS without Reinstalling, then here is a Nice Tutorial to change the Vista Serial key without reinstalling:

1. Click Start > Control Panel > System and Maintenance > System.

2. Under “Windows activation” section, click on the Change product key.

3. Enter the vista product key that you have obtained from Microsoft, in the change Product key dialog.

4. Restart Your PC to take Effect.

Don’t forgot to activate your Vista Key from Microsoft Site. Also take a look @ How to Change Windows XP Serial Key without Re-Installing