If I'm right, and as per your error message, PM assembler is build in 16 bit, and Vista 64 can't cope with 16 Bits. MPASM assembler (which come with MPLAB) should be build in 32 bits, this is something Vista 64 should handle.

Well, that's my own and personal understanding of it. I never use PM as assembler, but MPASM.

I guess one day Melabs may release PM in 32bits, or just suggest to use MPASM instead OR have MPASM installed in.

In MicroCode Studio, click on View > Compile and program option >> Assempler and check 'use MPASM' box. That should do the trick... finger crossed.