If after running the AVG removal tool oldman suggests you have any problem I would suggest a clean reinstall of avast.
Download the latest version of avast
http://www.avast.com/eng/download-avast-home.html and save it to your HDD (if you didn't save the original download), somewhere you can find it again. Use that when you reinstall.
Download the avast! Uninstall Utility,
find it here and save it to your HDD.
1. Now uninstall (using add remove programs, if you can't do that start from the next step), reboot.
2. run the avast! Uninstall Utility, reboot. If step 1 failed it may be necessary to run this from safe mode, once complete reboot into normal mode.
3. install the latest version, reboot.
Spybot S&D and AdAware even if installed shouldn't cause that kind of error. If you used windows add remove programs to uninstall them they are not so bad as to leave remnants, but that is something we can look at later if necessary.