It's hard to say what the issue is. It could be as simple as an outdated driver for your BluRay burner or fragmentation on your hard drive.
So naturally, go to Windows Update and ensure you have even the optional updates (Unless you've got Vista SP2 and you regularly manually update, it make take several trips to Windows Update), go to the Apple site and download the latest Quicktime and then give your drive a good defragmenting.
Try creating a short five minute practice file and see if you can get that to burn a disc. (I know BluRay discs are expensive, so it might be worth buying an RW for these kinds of tests.)
It's much easier to troubleshoot a five minue project than a 90 minute project, after all -- and it won't take nearly as long to render!
You could also disable Vista's User Access Controls and terminate any unnecessary processes (especially anti-virus products) until the burn has finished.
Insanity is hereditary, you get it from your children
This ARTICLE has tips along the line of Neale's suggestion. I use these for all editing and authoring sessions.
Going back a bit in time, it used to be standard practice to NOT use any other programs, while a burn was in progress. With newer, faster computers, this sort of passed by the wayside, especially with larger buffers, but might be worth the down-time, to see if perhaps too many resources are going to other operations, being performed during the burn. Just go grab a sandwich, while the computer does the burn, or do it overnight.