How to rollback a Service Pack in Sql Server?

For every service pack there is a README file supplied which explains the steps to ROLLBACK actions, in some cases of SQL 2000 it is not easy to do rollback which required a complete reinstallation. So for further information look at the each service pack release file for README.HTML file.

I don't think there's a short answer for that.

There's a Microsoft Support article about it:

http://support.microsoft.com/kb/314823

Why do you want to rollback a service pack that was installed?