If you simply want to repair your existing installed O/S then I highly recommend doing that BEFORE giving up and simply reinstalling your O/S. Repair rather than replace is always preferable unless the O/S is completely beyond redemption.
If you do a clean install you will lose ALL your current program settings, personalized aspects, as well as any program you installed for which you don't have the original installers available. Yes you can backup some files such as e-mail databases, contacts, bookmarks, music vids etc. and drivers (use the free Double Driver
http://www.boozet.org/dd.htm to backup/restore drivers for a pain free driver backup solution) However backing up/recovery of installed program settings is virtually impossible as such setting are invariably held in the registry.
To repair your O/S use the tools Windows provides that address O/S issues such as the System File Checker to address System file issues. As well the Recovery Console available when pressing F8 at the Windows boot logo has a number of options to address O/S problems. To invoke the SFC open an Administrative command prompt and type
sfc /scannow + enter. Allow SFC to complete (do not cancel it) to have system file corruption addressed. If you have a laptop, make sure it is plugged in as losing power/cancelling the process can create even more problems. You may have to run it more than once as not all issues may be fixed in a single run.
Take care