Assuming you have already uninstalled AVG via Control Panel/Add or remove POrograms.
Try the AVG removal tool which is here: http://www.avg.com/ww-en/utilities - careful there are 2, I would assume the 32-bit one is what you want.
Or go into the Registry and look for the AVG uninstall key that may still be there preventing the McAfee installer as follows:
Go to Start/Run and enter regedit hit the Enter key. Go to: HKEY LOCAL MACHINE/SOFTWARE/MICROSOFT/WINDOWS/CURRENTVERSION/UNINSTALL Look for any entries in that key that pertain to AVG and delete them. Be very careful to only delete those entries and none of the others. Try your install again.
Try running the AVG Removal tool as suggested by Ex_Brit so it should take care of the problem, but even after if you are unable to install McAfee suggesting you to contact the McAfee Technical support because there are some steps have to be done in the Windows of your computer to get this resolved.
McAfee Technical support – > 1-866-622-3911