First, upgrade to 5b, not a.
Second, you don't provide info about your capture hardware, IRQs, any any camera or external hardware you might be using.
Crashes are generally caused by resource sharing, assuming there is nothing wrong with the application. If you've installed other tools such as freeware, a few Pinnacle products, and a number of other software tools after you've installed Vegas, you could have corrupted the Vegas app.
Non-OHCI cards, some video cards, video acceleration in your graphics display, shared resources, bad OHCI cable...all these things can cause a crash.