This problem was mentioned before on SO in 2010, the solution was then to run the VB6 IDE in WinXP SP2 compatibility mode (as administrator). This solution did not work for me. It seems that WinXP implemented RegFree Com differently. So registering with regasm using the /tlb directive is now necessary.
Bob warns against the practice of using manifests for the VB6.exe to test Com wrappers for Net assemblies. We have used this technique for over a year without any problems though.