Possible corrupt files. Go to start/all programs/accessories, right click on commend prompt and select run as administrator, type sfc /scannow this will check system files.
possible failing hard drive, scan hard drive for bad sectors.
Could be something that has been install/uninstalled over the time that has corrupted the registry. You could do a clean install back to factory default this will remove every thing and you will need to reinstall all updates, programs etc. remember to back up all of your documents first.