Please uninstall AVG and then uninstall Avast. Having both on the system is not recommended practice. After that ( and rebooting your system after the uninstalls), please run the Windows Disk Cleanup Tool ( Administrative Tools > Disk Cleanup ). Allow the utility to scan your C: drive and make sure that Temp files are checked when you have the utility clean what it finds.
After this, you should be able to install either Avast or AVG once again.