Usually antivirus won't cause this kind of problem. You are likely having a line issue. Just to be sure you have tried your connection from another device, such as a tablet, smartphone or another computer?
Do you have a landline? If yes, do you hear static, humming or buzzing in the line during silent times in a call? Often these noises are signs of trouble that will degrade and disrupt DSL service.
Thanks. Once you can resolve the noise issue, you should find the DSL will stabalize. If your line was "optimized" to a lower speed, you should find the speed improving. A good way to start off is to research how to perform a NID test, and how to correct home wiring issues if all is clear at the NID.