Registry health = registry issues (number and severity of registry errors)
PC health = file issues (especially number of found temporary files)
After running 'Clean and Fix My Computer' in normal mode and fixing/removing ALL found issues, the health indicators should be set to 100%.
Look at this posts:
http://www.macecraft.com/forum/viewtopic.php?f=28&t=4522&p=29133&hilit=PC+Health#p29133
http://www.macecraft.com/forum/viewtopic.php?f=28&t=4517#p28960
Warning: as always, take care when you run this tool and before fixing/deleting take a look at the list of shown issues. Also always let the program make backups.