Boot into Windows 7, and go to Disk Management with both drives attached (right click Computer>Manage>Disk Management), then check to see if you have a hidden partition there with no drive letter allocated. Allocate a drive letter by right clicking the drive symbol at the top of the page and select "Change drive letter" then follow the steps and just use the first drive letter that is offered. Do NOT format when that is suggested.
Now boot to Windows XP and from Windows XP follow the steps in this guide. Although the guide is for Vista, the process is exactly the same: unplugged-xp-drive-for-vista-setup-can-t-dual-boot-to-xp-t84455.html
Let us know how you go or have any issues. By the way, read through this thread for some info on what you are up against, particularly Section 1. under "Prepare your system" as I suspect that's why you are unable to get your dual boot working: install-windows-xp-in-dual-boot-with-pre-installed-windows-7-t104890.html
"The great majority of mankind are satisfied with appearances, as though they were realities, and are often more influenced by the things that seem than by those that are." - Niccolo Machiavelli