Dual Booting between Windows XP and Vista

Step 1 Install XP
Step 2 Install VISTA

Never do it the other way around.

Windows installs a boot manager window which looks after the whole process.

Sometimes however things can go awry and you can lose access to one of the OSs.

This programme, BCD, helps resolve these problems brilliantly:

http://neosmart.net/dl.php?id=1

EasyBCD is NeoSmart Technologies' multiple award-winning answer to tweaking the new Windows Vista bootloader. With EasyBCD, almost anything is possible. Setting up and configuring Windows boot entries is simple, and there is no easier way to quickly boot right into Linux, Mac OS X, or BSD straight from the Windows Vista bootloader - on the fly, no expert knowledge needed!
EasyBCD is geared for users of all kinds. Whether you just want to add an entry to your old XP partition or want to create a duplicate for testing purposes; if you're interested in debugging the Windows Kernel or septuple-booting your seven test operating systems, EasyBCD is the key.
EasyBCD is used and recommended by Microsoft, PC-World, PC-Magazine, Softpedia, PC-Welt, and dozens more! In the words of Harry McCracken (VP & Editor-in-Chief of PC World): "[EasyBCD] impressed the editors with its functionality and usability. These are some of the most well-crafted services available - congratulations to EasyBCD!" Don't get left out – download it today, it's free!!