Hello zaina,
As far as Activation is concerned, you do not need to do anything before reinstalling.
After reinstalling, you will get a pop-up reminder to Activate. Click the reminder and the Activation Wizard will come up. Follow the directions and Vista will be Activated.
If you have any additional Activation related issues or questions, please post into the "Install Update & Activate" section of the Vista Answers Forum:
http://social.answers.microsoft.com/Forums/en-US/category/windowsvistaThank you,
Darin MS