If AVG has been installed for some time and has been working correctly up until now, there are multiple possibilities...
If you think your computer has been infected by malware, then you should follow the instructions for posting a DDS log
in the Malware Removal room.
If your computer is otherwise operating normally and you have no reason to suspect a malware infection, then the issue may simply be a problem with AVG. If that is the case, you may wish to consider selecting and downloading a different free antivirus program such as Microsoft Security Essentials
. After downloading the new antivirus, uninstall AVG, restart your computer, and then install the new antivirus.
If you encounter difficulty removing AVG, then you can use AVG's AV removal tool for 32 bit systems
or AVG's AV removal tool for 64 bit systems
If you are not sure whether you have a 32 bit or 64 bit system, just go to Control Panel > click on System and a window will open with information for your computer's operating system.
If your preference is to keep AVG, and you do not feel your problem is related to a malware infection, then you may wish to troubleshoot your AVG problem at AVG's forums