Thanks for your response.
I'm at work at the moment, which is a Windows XP Pro machine, I downloaded the (free) SDK and gave your example a go, and it definitely "breaked" at the a% = 1 line so I now know what to expect.
I'll try the same example at home tonight and see how it behaves, but even in my simplist of programs it has failled to pause execution.
Does anyone else on here with Vista x64 (or any Windows 64 bit) have the same problem?