Windows XP to Windows 7 Start-Up

I am having a problem with my computer start-up. I upgraded my computer software from Windows XP to Windows 7 and now there is a problem with the start-up. I tried starting the computer in safe mode, but this isn't working either. When I tried rebooting the computer using the CD Drive and the Hard Disk, the computer runs continuously. I decided to return to using Windows XP, but this is not an option. I have tried all I know how to do. Your help with this matter would be greatly appreciated.
