If you were running the 180 Eval edition then you can go a SKU upgrade to Standard Edition. Put the DVD in the drive, and cancel out of the installer. From the Start > Run, run d:\setupe.exe /SKUUPGRADE where d is the drive letter of the CD/DVD drive.
If you have the actual Enterprise Edition installed you will need to uninstall and reinstall SQL Server.
In either case after the installer has finished you will need to reinstall the service pack and hotfix that you were running before.