Blaster and Sobig caught a lot of people out, one theory is that a lot of people have now picked up on AVG free and are taking belated precautions, so the server is now overloaded. No way of checking this though. Rumour has it that AVG 7.0 will soon be available as a free version, this has incremental updates so should help.
If you can't wait try avast! 4 click here best free anti virus according to virus bulletin and my favourite. Already has automated incremental update system.