Problem arises after upgrading Windows XP to Windows 7

Hi,
I have just upgraded to Windows 7 from Windows XP. The computer has begun freezing within a few minutes after it starts up. It asks me to choose whether to boot into safe mode or not.
Unfortunately, it cannot boot into safe mode, or use anything else in the menu. I tried using the CD drive and hard disk to no vain. Consequently, I cannot downgrade to XP.
Has anyone experienced such a problem?
What could be the solution?
Thanks in advance.
