Whilst I don't know if any sort of conflict (with a previous AV or remnants) might cause this type of effect, it is worth ensuring AVG is completely removed.
- Ensure that all remnants of AVG are gone - AVG8.x (or higher) Remover, download tool from here,
http://www.avg.com/download-tools there is a 32bit and 64 bit windows version, ensure you use the correct one.
The corruption error from avast is certainly a possible indication of a conflict, so perhaps the above removal tool might help, followed by a clean reinstall of avast.
- Download the latest version of avast, 5.0.594
http://files.avast.com/iavs5x/setup_av_free.exe and save it to your HDD, somewhere you can find it again (if you didn't save your last download). Use that when you reinstall.
- Download the avast! Uninstall Utility, aswClear5.exe
find it here and save it to your HDD (it has uninstall tools for both 4.8 and 5.0).
- 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 from safe mode, first for 4.8 if previously installed and then for 5.0, once complete reboot into normal mode.
- 3. install the latest version, reboot.
No problem, glad I could help.