To confirm: there are two separate drives, correct? Not two partitions on the same drive? (this is important)
The boot files necessary to boot both operating systems were on the XP drive. When you reformatted the XP drive, all these boot files went away. Now, no operating system can boot (there is no boot sector).
This can usually be fixed fairly simply:
Just boot off your install disk, and choose "Repair your computer" in the bottom left of the first install window (don't click "Install now", look in the bottom left corner of that window for "Repair your computer"). The repair utilities will scan your computer for installations of Windows 7. It should find your installation and say something to the effect of: "A Windows installation has been recovered. Please reboot to fully recover this installation....blah blah" Allow it to reboot. Boot off the install disk again, "Repair your computer" again, and this time, your Windows 7 should show up in the list. Click next. Choose "Startup Repair" at the top of the list. The repair utilities will scan for problems and fix what is necessary to be able to boot. Reboot again. You should now be able to boot. If not, simply do the "Startup Repair" one more time (it sometimes takes two tries to fix everything), and it should almost surely be correct now.
Does this solve your problem?
If not, tell me exactly what you did and what happened when you did it, and I will do my best to correct the problem.
Edited by keyboardNinja, 01 February 2010 - 01:37 PM.