Hi,
Windows Developer Preview setup modified your BCD settings so that Windows Developer Preview is the default. You can change this back to using Windows 7 as the default using one of two methods:
1) Run msconfig.exe and use the Boot tab to change the default.
2) Boot into your Windows 7 install and run the following from an elevated command prompt: bcdedit /default {current}
Windows Developer Preview setup should not have modified your Windows 7 install as long as you installed to a different partition or drive.
Hope this helps,
Richard