William's How-To on this forum will tell you how
I just finished dual booting a Vista notebook with XP. William's notes were invaluable!
Try not to be put off with all the posts after the initial ones. I was having problems in getting Vista to boot after I had installed XP. I was getting the /ntldr not found problems.
I perservered and I now have a dual boot system working just fine.
I ended up using VistaBoot Pro 3.3 to fix my boot loader problems - The Vista and XP ones are different and were fighting each other!
At one point I thought about shooting myself! But keep at it - it can be done.