Okay, so what happened was.....when you installed the Trial version of 14......it overwrote to the directory where 10 was stored.
Unfortunatley, that corrupted the 10 files.
So.....you have to Uninstall 10, and then REinstall it.
Or, if its an option......try to REPAIR the 10 install.
Go into your Prgrams and Features (in the Control Panel) and see if you can repair the 10 program.
Select it, and up at the top you should see Uninstall and/or Repair.
Or, the Uninstall program might have a repair tool.
How did you initially install 10 on this computer?
Did you have the discs?
Did you buy it online, and then install it from a file?