Everything you need to reinstall the licensed Win7 on your machine is included here including the latest official ISO download with SP1 and the MS tool to write it to flash stick: Clean Reinstall - Factory OEM Windows 7
Since you had GRUB on the HD I would wipe it first using Diskpart Clean Command
from the booted installer's Command line accessed this way: DISKPART At PC Startup
If you follow the steps to install and maintain Win7 with only the methods and tools included you should have no problems with Win7. If so post back and we will help you with Troubleshooting Steps for Windows 7.