If all the data you wish to preserve is NOT on the "C" or Windows installed drive (or partition) then a reinstall should not affect them so long as you don't select one of them to reinstall Windows on during the reinstall. Before you do that however what other methods have you tried to sort it out. I'm not sure if there is a Windows 8 update troubleshooter. Have a look on the MS support site.
Forza Horizon 4 review