I just installed on mine and part of the process is to do a full scof the system to ensure no existing viruese. As long as you let this happen the system should be clean ( some viruses disable your avg ).
If it didn't get all the latest upgrades and do a full scan which took 15 mins on my machine.You do this by double clicking on the icon in the sys tray.
If that all goes ok then doublr click on the tray icon and enable auto protect. I should run next time you start windows.
hope that helps