How to downgrade from SQL2005 Enterprise to SQL2005 Standard in Windows Server 2003?
Unfortunately there is no easy path to downgrade. You will need to:
You may be able to make the process quicker by copying your existing databases to another location, doing the re-install and then copying the databases back, overwriting the existing DB's, including the system DBs. However, make sure you have a good backup off all DB's as well in case this does not work as you want.
As Brent said, your also going to want to check that any of your DB's are not using the features of the EE edition.
I'm afraid the only possible solution is to backup all your databases, uninstall SQL Server 2005 Enterprise, install the Standard version and restore the databases.
Don't forget to script any user objects of your master database and other system databases (job definitions etc.)