Afterburner has no affect on internet speeds or anything of the sort. It can't do anything unless you run the program, normally it won't run at start-up, but you can make it do that by ticking a box. All it does is allow for GPU overclocks and provide benchmarks, you might have had your card set on unstable settings.
Re-install Afterburner and revert to default settings. This is easily done by the reset button, right next to the settings button. If not, then manually move the sliders until you get the below settings. But you shouldn't need to, the reset button resets any overclocking. Make sure the program doesn't start-up on boot, you don't need it. Even if overclocking, the overclocking settings can be saved and set to always run that way on boot. Which it sounds like it currently is.
You'll know it's reset because the bars will be on the folowing settings:
Core Voltage = +0
Power limit = 100
Core Clock = +0
Memory Clock = +0
Fan Speed = Auto
In all honesty, you shouldn't let folks mess about with your settings, especially if you don't know what it does or if they don't bother explaining it to you.
You can ask and find out about overclocking much better in the owners thread for the respective GPU or the Overclocking and Cooling section of the forum. Retailers are not always going to help you out if your card gets busted from overclocking, cards are always sold at stable settings and any further adjustments are the responsibility of the user.
It is also possible that you got a dodgy card in the first place, in which case you should have contacted the retailer first and not this random person who 'messed around with settings in MSI Afterburner'. They'd usually help you go through RMA and/or get a refund/replacement.
