If your hard drive gets full, PostgreSQL will become corrupt. This will also cause performance issues. You should keep at least 10% free space at all times. Here are some suggestions to recover disk space on your hard drive. If you are moving large amounts of data, such as creating a backup, Windows also suggests you maintain an additional space twice as big as the data you are moving for file swapping.

1. Run Database Maintenance to reduce your PostgreSQL database size.
2. Make sure that PostgreSQL is not saving log files and delete them if they exist.
3. Run Windows Disk Cleanup Tool. (External Link)
4. Remove movies, videos and programs you no longer use.  These can take up a lot of disk space.
5. Clear the recycle bin on your PC.    
6. Run WinDirStat to see what size each file and folder is to determine where you may be wasting space.
7. Consider turning off Hibernation (external link).
8. Buy a new Hard Drive increasing your available disk space.