|The old try dual boot and bork your system trick ehhh?|
Sometimes the bios kind of confuses the deal. Some sata and add-on cards might be one way to the bios and another way to the OS. First bootable active partition issue is my guess no matter what the reason.
Xp borked your install of vista, as with most MS products. Pop in your Vista DVD and use the repair feature. Hopefully it will fix your Vista install.
Then get either virtualbox or virtual pc and run a vitual machine or re-read the hundreds of how to's on dual booting Vista and XP guides.
"Best Practices", Event viewer, host file, perfmon, antivirus, anti-spyware, Live CD's, backups, are in my top 10