Backup and Reinstall
Backups and reinstalls are one of our most frequent duties and 90% of the time will be somewhat time-consuming yet relatively painless. The key here is to double-check and triple-check before you click Enter that last time and wipe the drive clean. As long as you know the files are backed up (or in some cases, don't need to be backed up), you can essentially do whatever it takes to get the machine working.
Measure twice, cut once. That is the absolute simple truth of this process. So to make sure you measure correctly in the first place, do yourself a favor and take steps to be certain of what needs to be done before and after the reinstallation. For the simple, pre-made, really-no-one-to-blame-but-yourself way, take a look at the document in the link below.