I had a problem registering a 32bit dll on a 64bit server.

The dll was custom written and replied upon some registry settings.
For example: HKLM\Software\Jigsaw

Turns out that if it's a 32bit dll the registry settings need to be under
HKLM\Software\Wow6432Node\Jigsaw

Moved the registry settings here and regsvr32 then worked.