Removing Dell Dimension 4550 Motherboard

Hello,

Due to my kids constantly plugging things in and out of my USB ports 2 are now broken. I wanted to remove the motherboard so that I can replace the ports. I removed all the screws that I can see holding the board to the case, but it still is being held in by something else. Any help would be appreciated.

Dell 4550 Manual

I too have a Dell 4550 & I also know how hard it can be to navigate thru the Dell site But if you click on this link http://support.dell.com/support/edocs/systems/dim4550/
& select Replacing the System Board You will see that the mobo slides out on a tray.
Look for a green tab to slide it out then once out one would remove all the screws & then give it a lil push to the left ,kinda like you would on a side cover of a tower.
Good luck I hope the link is of some help!
PS: I need a replacement board for My lil girls 4550 since a power surge fried it & it's no longer covered be Dell.