Windows 7 Startup Repair and Reinstalling?

So my brother's computer is stuck in that Windows Startup Repair Loop where keeps on saying checking/repairing problems and its been like that for a long time. When my own computer had that problem, I just manually turned off and started my computer to make it work, but on my brother's the startup repair keeps on showing up. I looked around and I read that reinstalling the Windows 7 CD will fix it but will it erase all his personal data and such? or is there any other way to fix the problem?