It could also be that you're infected. Many viruses, trojans and worms actually block your antivirus from downloading updates, or even working. And many will also block you from downloading new antivirus programs. See if you can do AVG's internet scan (not the program download but an actual virus scan), or McAfee's online scan. http://us.mcafee.com/root/mfs/default.asp
They will usually detect viruses and many other problems fairly well. You can also try downloading Spybot Search and destroy and run a scan. http://www.safer-networking.org/en/index.html
This will often pick up other problems. If you're not comfortable, then you may need to have a professional look at your system. There is a lot of trouble shooting you can do on your own but you have to be comfortable getting into the "bowels" of the system so to speak.
Sometimes things are running in the background and you have to stop them before any software can get rid of them. Also make sure you have all microsoft critical updates and that you have automatic update running for microsoft (if you're using windows.) Those usually include the mallicious software updates, which helps protect your PC.