I remember looking for Visual Basic decompilers/disassemblers a long time ago to no avail. There may be things out there now though. The most I could come up with was PE Explorer: http://www.heaventools.com/overview.htm

Have you suggested running your application under Windows Compatibility mode: http://www.sevenforums.com/tutorials/316-compatibility-mode.html

Edit: Further Googling: http://www.w7forums.com/visual-basic-3-0-compatible-64-bit-version-t5146.html

I maintain legacy 16-bit VB3 software that is compatible with Windows 7 and have done a lot of research in this area.

16-bit applications will not natively run in Windows 7 64-bit full stop. This is because when a 64-bit capable CPU is started in 64-bit mode, it cannot change into 'real mode' (the mode used by 16-bit applications) without a hard reset. If a processor is in 32-bit mode, it can swap between real mode and protected mode at will.

The only workaround here is a virtual machine- Windows 7 comes with a free version of Windows XP which is usable inside a custom version of Microsoft Virtual PC. Google for "Windows 7 Virtual XP Mode". Once your copy of Windows 7 has been validated as authentic, it will allow you to download Virtual PC and the XP Image.

It's definitely not a pretty solution, but the only option for my customers who have gone ahead and bought a 64-bit OS without checking to see if their installed software is 64-bit compatible.