Why does the desktop Hard drive icon keep moving?
It looks like your hidden .DS_Store file in your Desktop is corrupted, which will cause this issue for sure. 

You can restore this by using Terminal. 


Open Terminal.app
Type cd Desktop
Type ls -aF
Now, if you see a list of all your Desktop items, you're in the right place. And make sure you check whether or not the .DS_Store file is there
Now, type rm -i .DS_Store
When you're prompted hit 'y'
Log out and in again


Mac OS X should have rebuild this file and your icons should be ok. Let me know if you have any issues with the steps, I'm glad to help you.