Win 2003 32 bit will work quite happily on a 64 bit box, assuming it's x86 compatible hardware (as opposed to Itanium).
However, you might as well find out what the problem is with Win64. You don't say which process is trying to load the dll, nor which dll you are trying to load, nor which version of regsvr32 you called (there are two versions on Win64).
If your process is 64 bit, make sure: - The DLL you're referencing is 64 bit - You called the 64 bit version of regsvr32
If your process is 32 bit, make sure: - The DLL you're referencing is 32 bit - You called the 32 bit version of regsvr32
Sorry I can't be more specific; I have neither direct experience of Win64, nor of ActiveX, so it's the blind leading the blind :-)