I want to shut down Avast temporarily, in order to run a program called “ComboFix”. However, when I go into Task Manager/Processes, and click on any of the Avast Services, it does not let me shut it down. Please advise. Thank you.
Right click on the orange avast ball and go to avast shields control and choose how long you wish to disable, remember to turn it back on 
If your not under the instruction of a malware expert you probably shouldn’t be running combofix.
I would be more inclined to ask who suggested you run Combofix ?
This is a powerful tool which really should be user under guidance of a malware removal specialist. If that is the case then disabling avast (as craigb mentions) until the next reboot would be the option to choose as you would be normally rebooting after running combofix.
quote Bleepingcomputer
[b]You should not run ComboFix unless you are specifically asked to by a helper[/b]. Also, due to the power of this tool it is [b]strongly advised[/b] that you do not attempt to act upon any of the information displayed by ComboFix without supervision from someone who has been properly trained. If you do so, it may lead to problems with the normal functionality of your computer.
To put this into simple language:
Using Combo Fix without proper guidance or knowledge can leave you without a usable computer.