No.
Just use OOBE (Out-Of the-Box-Experience).
1. Press the Start button.
2. Type sysprep.exe and then hit enter.
3. Make sure it's set to "Enter System Out-Of the-Box-Experience" and the second line is set to "Shutdown."
4. Hit okay.
It should shutdown and boot from the beginning. It will make you create a new account as if you just turned on your computer for the first time. After you make another account, you should be able to access your old account with all your files intact (excluding the start menu from having all your program shortcuts.) When you check system properties though, it will say that you need to activate windows. Just use the serial code that is on the side of your case and you should be all set.
I believe you can only do this once with an OEM copy of Win7 and there is a chance you may lose all your data, or have to contact Microsoft for another key. Fortunately for me, I did it this way after ordering a new mobo and it works perfectly fine.
If you can't find sysprep through the search bar, then just go to Computer\Windows\System32\sysprep and go from there (sysprep.exe). You can also use Google.
So again, you DO NOT have to purchase a new copy of Win7. Just make sure you back up your data, just in case.