So you tried to install OSX/a distribution of linux/your own homebrewed OS and now you can’t get your old vista partition to boot?

Throw in a vista setup CD and boot from it.
Once setup loads, hit the next button to select the language settings.
Then click on the link in the bottom left to ‘Repair your computer’
First run the startup recover tool if that doesnt work open up the command prompt and type:
bootrec.exe /fixmbr

Should fix it for you.