if you're comfortable using gimp i suggest sticking with that, if it wont run here's what you do:
(This is for Windows)
Goto Control Panel> Programs and Features
Click on gimp and click 'Remove'
Goto your gimp directory and if there is anything left,delete it.
You should then be able to re-download and re-install Gimp. If this doesn't work, i'll find a better Photoshop tutorial for you.