Vista will not boot

Try loading your vista disc and booting from that, there's a repair option that is supposed to tell if anything is preventing Vista from loading. Other than that, you might need to reinstall.

Did you just install SP1??? Some machines were having a problem with SP1 if you had previously installed a KB hotfix that interferes with SP1. Try booting with your disc and doing a system restore to a earlier time when everything was working properly. Also, the option per Ohiou above is available.