Reinstalling drivers...

So I'm a complete noob and was playing around with the settings. I screwed things up and ended up having to delete my drivers so now my ATI Radeon HD 5770 won't even show up on the device manager. How can I reinstall the drivers and everything? I nee d a quick response please.

edit: it's back on the device manager but I cant get back to my native resolution (1920x1080) because it isn't listed...

"Windows cannot verify the digital signature for the drivers required for this device. A recent hardware or software change might have installed a file that is signed incorrectly or damaged, or that might be malicious software from an unknown source. (Code 52)"