You were not being cautious if you did the operations with no way to recover as specified.
Even in clearcut cases like this where the old OS should come out with no hitches, there can be reasons why it bollixes the MBR on the other partition. The solution is easy and automated but requires the Windows 7 DVD or repair CD I clearly warned you about to run Startup Repair.
Now, make a Windows 7 Backup Image stored externally or to a primary formatted partition on a secondary drive, so you never have to reinstall Windows 7 again. Plus after the image is saved in 20 minutes, it will offer to make you a Repair CD which you should keep close at hand.