Hello mate.
I'm just going to put this out there for you to consider; I would suggest a complete wipe of the Hard Disk Drive to remove Linux/Grub but that will over-write all data on the HDD for good.
After you have
copied out or made
back-ups of the data you need to save to external media; if you can find an exact Windows version that the PC shipped with you can do a
clean install using the activation key on the COA sticker attached to the PC though it may require a robo-call to MS to sort the OEM batch-key.
How to Activate Windows 7 by Phone
If you can't find an exact same version, you could use the info in this tutorial to create an "all versions" installer of any version Windows installer, to install using the same method I posted above.
Windows 7 Universal Installation Disc - Create
After you have made backups of everything you care to save, the best method is to do a complete wipe (
secure erase) of the entire Hard Disk Drive first, it over-writes everything, all the old Windows code including all the old drivers/programs, giving you the best possible space to install Windows to.
SSD / HDD : Optimize for Windows Reinstallation
