If you upgraded your motherboard to a whole different chipset forget about booting back into your old system. You need to do a full wipe and reinstall (Recommended) or an upgrade install. Either way you need to boot from the CD which when the PC first starts up you will see the Splash Screen. Hit F11 a few times and it should come up with the Boot Screen. Toss in your Windows 7 Disk and then select the DVD rom Drive. IF you don't have a way to get your data off i would just do an Upgrade install (Which will keep all your data and programs) and then Backup everything, and then do a Custom Install next time and then delete all the partitions when you install and then start fresh. Upgrade installs always leave files of the Old System or Old OS if you upgrade OS's and can only cause issues in the future.