Opening a Compaq Evo case

[saheton]

well, i have this compaq evo right, and i want to get the data off my old HDD, onto my external. The plan was to unplug the CD drive, pug in the HDD and boot the computer, then i'd just copy-paste all of the stull onto a .rar archive on my external. BUT even though it has those tabs on the side, it eont budge, i read that you need to unscrew those screws at the back, but the have these six little "v" instead of a normal head. please help. i will supply photos if needed.