Hi!

This is what happens when I press "F":

It's not a big deal, but it's a handy key. 
Is it easy to fix it?

Thanks in advance!

It looks like the debug keys were enabled in the start.ini by default.  'F' is programmed to view the wire frame.  Open the start.ini in a text editor, scroll to the bottom, and disable debug keys by setting ondebugkeys to zero:

[controls]
ondebugkeys = 0

It worked! Thanks you :D