I like AVG as well as Avast. Both are good, but do lack ability to remove or secure some viruses. However if you allow it to auto-update on its own, it should only take 2-3 times to catch it. Also some viruses will not be seen until after a re-boot of system in which Avast will scan system on boot up.
I had the issue with Anti-virus 2009 last year, and it disabled avast from fixing it, but avast did reboot the system in which it was able to remove before XP booted up.
If it comes up in the blue screen and starts scanning the system, DO NOT BYPASS IT! Go grab a cup of Joe and enjoy a couple hours likely. the system will recover and boot on its own once it is done with scan. If it finds conflicts though it may request input from you as to how you wish to proceed. I reccomnd to always allow it to do what it needs and not skipping a scan.
My system had become so possesd that I could not log on to it or otherwise access any help, I spent a month manualy removing code and doing my best to not lose anything, as all of my system disks were half the country away. 6-8 hrs a day for a month and minor data loss was a good month!
I hope that helps you.