If you install it agin everything will go back to normal, and you'll get your dual-boot back. This sort of thing happens all the time when you reinstall an older version of Windows. I very much hope that when Vista goes gold it will have a better bootloader arrangement than they've had so far since NT was introduced.

I'm planning on installing Build 5342 64-bit this weekend if I can get it downloaded in time (10 hours to go as I type this). Then I'll have a triple-boot MCE2005, XP Pro and Vista setup