Download an ISO here, be sure to select the correct one for your setup.http://www.mydigitallife.info/official-windows-7-sp1-iso-from-digital-river/
Note this is not piracy by any means as you own a copy of Windows 7 already.
If you end up having to completely reinstall it to fix the issue it will require you to enter your key before proceeding, so I hope you have it noted down.
It should still allow you to use the repair function without it.
Burn it to a disc using ImgBurn here: http://imgburn.com/index.php?act=download
Or make a bootable usb stick using this: http://www.isotousb.com
Note that if you're using a different PC to make the usb stick, the antivirus might give an alert on the isotousb site - ignore it, it's a false positive. I use it all the time.