Here is a way how you can fix this problem without having to reinstall the OS again.
Note : If u have more than one hard disk then you may want to connect only the hard disk which has windows OS on it.
1)Put the windows DVD/CD in drive and boot with DVD/CD and let the loading take place(It may take around 5 mins)
2)After Some time you will get a window as shown above, just click on next
3)Now click on Repair your computer
4) Click on next.
5)Now click on Command Promt.
6)The command prompt will show up, Here just type bootrec /fixmbr .
(There is a space between bootrec and /fixmbr)
And you are as good as new!!!!!!!!!