I can't find my original CD (probably in storage by now) and I need to move the game to another computer. I've moved the files, and I think I just need to create the registry entry but I figured I'd ask here if anyone has done this successfully. Thanks!

I've done it a few times. All you need to do is copy the UT2004 folder over to the new PC and set up the registry entry.

In older versions of Windows the key goes in in:

HKEY_LOCAL_MACHINE\SOFTWARE\Unreal Technology\Installed Apps\UT2004
Value name: CDKey
Value Data: XXXXX-XXXXX-XXXXX-XXXXX (Make sure to use dashes and CAPS when entering your key)
Value name: Folder
Value Data: C:\Unreal\UT2004 (your folder path)

In newer versions (64 bit)

HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Unreal Technology\Installed Apps\UT2004
Value name: CDKey
Value Data: XXXXX-XXXXX-XXXXX-XXXXX (Make sure to use dashes and CAPS when entering your key)
Value name: Folder
Value Data: C:\Unreal\UT2004 (your folder path)

Thanks Jefe! I did find your post on the Steam forums about this (should've looked before asking) Thanks though!