Avast is an anti-virus, CCleaner is a cleaner yes, we just use ATF cleaner here, it is not as drastic as CC.
You will not have to disable Avast because if you run AVG Anti-spy in safe mode as instructed then Avast will not be running anyway...but even in normal mode you can leave your anti-virus program running when running AVG...
Now the reason we want you to run it in SAFE MODE is because in Safe Mode the barest essentials are running, and usually if there is a trojan, or virus on the system which starts up when Windows starts it will not start up in Safe Mode. Many times the difficulty removing a virus or trojan is because it IS running and you generally cannot remove a program that is running, hence the run in safe mode instruction.
Before you boot to safe mode please update your Avast program. Then run a Full System Scan with it and quarantine everything it finds.
THEN disconnect from the internet and reboot to SAFE MODE.
Run ATF as directed in the sticky from PP. THen run AVG as the instructions give in the sticky. Be sure to remove EVERYTHING found.
Save the log.
Reboot to Normal mode and run a new HJT scan and post back with that new log and the AVG log.