My gut feeling is heavy infection with viruses. I am surprised you had the patience to wait 2 hours for it to boot!
Does it behave well after it is booted?
What happened just befoer it slowed down?
Has there been any new installations?
Has the machine been recently upgraded to Windows 7?
What are the specifications of your machine?
Is the data on the machine valuable to you?
As I suspect a virus could you install Malwarebytes, update it, and then run a full scan? I would advise you to remove anything it finds.