When you P2V (Physical-to-Virtual) Windows, it will want to re-activate Windows due to hardware changes. If you're physical server was running a OEM version of Windows Server, you may not have access to the COA Label (Certificate of Authenticity).

These keys from Microsoft can be used after the P2V to help activate your OS.

Note that the OEM license only covered the OS while it was on the old hardware. Once it's P2V'ed to a Virtual Machine, you'll need another license to cover it, but those keys (retail or volume license) won't actually work in the OEM version of Windows Server 2003, hence the list from Microsoft to get over that road block.