Same thing happened to me, older system, Windows XP, 14GB partition, 1900MB free.
Custom install, File & Network shield, English language. Would not install, same error as above.
Reverted to avast 2014. I opened a ticket with avast, asking if this was an installer error.
The response was unintelligible, asked me to run the default install and not to customize it at all.
Sounds as if the responder had not read/understood the point at all.