The first step is to verify which edition is currently in place (see KB article #229). If you are currently running the Desktop edition, then you may proceed with the following instructions to upgrade to the Standard or Enterprise Edition.
In order to upgrade the edition from desktop to enterprise, please follow Microsoft's instructions or Axon's instructions. You will need to have your SQL Server 2000 CD in hand before beginning the process. Please note that an upgrade to the Enterprise edition is a two-step process. The linked instructions allow you to go from the Desktop (MSDE) edition to the Standard edition in one step, but not directly from the Desktop to Enterprise edition. To go from Standard to Enterprise, you will need to repeat the entire process a second time.