|Ideally you should install XP first - as win7 install will then create a dual boot menu for you. The best way otherwise IMO is to use a third party boot manager (I use www.boot-us.com - there is a free version with restricted function, there are a fair few out there if you google). Using boot-us, you would install xp on the second drive with the first drive disconnected (very important that its disconnected). Once XP up and running ok, reconnect first drive & boot from it (system should default to first drive). Install boot-us on win7, then run it (need runas administrator) to create the boot menu. Can store the menu in several places - you'll see the prompt. If you store it in the boot sector or free partition, you'll just get the menu every time you boot machine (there is option in boot-us to remove the boot menu too). If you store it on a floppy (probably don't have one!) or a CD (need to burn the image file it produces), you'll get the menu if floppy/CD in the drive and machine is set to boot from it. This method leaves both operating systems independent, so problems with one will not affect the other.|
Another possible approach is virtual PC (or other virtual machine software, like virtual box/vmware). Here you install XP in a virtual machine that runs under win7 (win7 professional & Ultimate versions have a special win7 version which will integrate the xp so that apps installed on xp can be run directly from win7. Other versions you'd need to use virtual pc 2007 or one of other virtual machines.