Scott Guthrie has leaved a feedback here explaining the reasons of the VS2003 incompatibility with Vista. The content is interesting and must be in evidence:
"The big technical challenge is with enabling scenarios like advanced debugging. Debuggers are incredibly invasive in a process, and so changes in how an OS handles memory layout can have big impacts on it.
Vista did a lot of work in this release to tighten security and lock down process/memory usage - which is what is affecting both the VS debugger, as well as every other debugger out there. Since the VS debugger is particularly rich (multi-language, managed/native interop, COM + Jscript integration, etc) - it will need additional work to fully support all scenarios on Vista. That is also the reason we are releasing a special servicing release after VS 2005 SP1 specific to Vista - to make sure everything (and especially debugging and profiling) work in all scenarios. It is actually several man-months of work (we've had a team working on this for quite awhile).
Note that the .NET 1.1 (and ASP.NET 1.1) is fully supported at runtime on Vista. VS 2003 will mostly work on Vista. What we are saying, though, is that there will be some scenarios where VS 2003 doesn't work (or work well) on Vista - hence the reason it isn't a supported scenario. Instead, we recommend using a VPC/VM image for VS 2003 development to ensure 100% compat. Hope this helps - even if the answer isn't entirely what we'd all like it to be.".
So, what's the destiny? 2 machines on the desk or Virtual PC images for all. Bleah!! 