You may only have 20% of the 120gig left but avast will scan much more than this depending on the amount of archived files on your drive. Also the more archived files you have the longer it will take to scan.
see the attached example. I am storing appr. 29 gb on my C drive but Avast has tested 47 gb of data. Took 30 minutes to scan 47 gb.
The boot time scan is important for detecting and cleaning up a malware infested system before the malware is able to run, i.e. rootkits, etc. Otherwise, it may be difficult to stop these processes once they have started.
The only reason I performed the boot time scan is because I was testing Avast with different types of malware.