Windows 7: reinstalling does't genuin with same product key

hi guys first of all thanks to all making for such type of helping platform.Earlier i have installed windows 7 ultimate 64 bit original.Due to some reasons i have to format it.my laptop was not able to read that dvd so i used my friends dvd(win 7 64 bit) for formatting.now for activation i used my keys but it's not accepting and showing windows is not genuin.that original dvd is also not of use.i cheked microsoft site for direct downloading but all in vain.....................
So plz help me.Thank you

Use this tutorial :
Activate Windows 7 by Phone

** Go to Step 2 - Activation by phone
Make sure you stay on line (step #9 ) and talk to an activation operator who will help you.

Not to be the bearer of bad fortune but your key (and disk) may be a forgery. Did you buy it off Ebay? Check to see if your disc has a stick-on label rather than burned in holographics. If it's a label, it's fake.