Visual Mode Selection in Windows 7

I am using gvim 7.3 on Windows 7. I need to select the four x's in the following sample text. I positioned the cursor on the upper left 'x', then typed 'v' to enter visual mode. So far so good, but when I move the cursor to the lower right 'x' I lose the selection. How do I designate the selection area in visual mode?
 
Thanks,
 
Roy Fulbright

abcd01
efxx02
ijxx03
mnop04