Following simple tips of when you will use 2 operating system in your computer .
Ok, follow these steps:
Before all, download EasyBCD : http://neosmart.net/dl.php?id=1 .
1.Install your windows vista before windows xp
2.Take your Vista CD or DVD then boot , follow process when perform Startup Recovery
3.Restart, and Vista loads.
4.Install EasyBCD you which have download.
5.Now ,Run EasyBCD then goto "Add/Remove Entries"
6. Look at the version drop-down menu and select, Windows XP
7.Change the drive letter to one on which XP is installed, rename accordingly.
8.Take, "Add Entry" , then "Save"
Now, if you booting you will have two options Windows xp and vista, and you can choose one of that.