Why am I not surprised by this? Install the beta Office 2007, do lots of productive work with it, forget the trial is about to run out and proceed to lose most of the functionality. Ok, no problems, just reinstall an older non-beta version and keep working... Problem. You've saved your work in the new shiny XML based format that the old versions don't understand. Ok, you can still use the beta to open the documents, maybe you can re-save out in an old format... Nope, it expired so you can't do anything with it, they only gave you the opening ability in order to taunt you. Wonder how much productivity is going to be wasted accidentally with this?
Yes, I know, use Open Office =P Point is MS don't provide an easy solution to a problem of their own making.
Also, if you do decide to use Vista then keep an eye on your northbridge temperatures. Just a tip.
Problem unrelated apparently, but I'll still moan about Vista anyway :-)