Well this is how i fixed it. 

First, this happened to me right after it upgraded. I had not yet applied my Windows 7 Activator.

Upgrade went flawless, took about 4 hours and on the last boot i got a black screen with a blinking cursor. 

Here is what i did: First i rebooted to the windows 7 DVD. Clicked yst to the English prompt etc and clicked the repair option. I then opened up the command prompt option and typed bootsect /nt60 and then restarted. DIDN'T work. (You might try that first though).

Then i rebooted back to the cd and clicked on the repair startup option and it told me that it did not find any issues with my start-up. However, i ran that tool 3 times and that didn't work. (i still recommend you do this 3 times to see if it works for you).

Then i rebooted and went back to the command prompt and typed bootrec.exe /fixmbr (got a success message) and then i immediately following that typed bootrec.exe /fixboot and then restarted and THAT FIXED IT. 

Hope this helps.