Can't connect to the internet!

Hello,
One day my laptop(windows xp) decided to start going really slow, I assumed it had a virus so i downloaded AVG to run a virus scan..this seemed to make things worse and now my laptop will not connect to the wireless internet at all. When I try to connect it either says no wireless connections available or a warning pops up saying 'Could not connect to the specified wireless network due to the mismatched encryption settings. Change the encryption settings now?' After that I don't know what to do! Please Help!