After scanning AVAST told me that 18 GB was scanned but the size of my hard drive is 37 GB. I had the entire hard drive checked for scanning before I began. Why is this?
Hello 
Well, maybe your hard drive is not full and the used space is only 18GB 
:
avast! is scanning only the existing files.
Welcome to the forum, cool9.
Perhaps it is because your 37gb harddrive only contains 18gb of information? If the other 19gb is empty, that part of your harddrive will not scanned since there is nothing there to scan.
It depends which scan mode you use and if you include Archives.
Normal and Quick modes skip filetypes that cannot be infected or are not infectable.
Thats why scaned size is smaller since avast! skips those files which aren’t counted in that size because they wre skipped).
Thorough mode will scan everything on the other hand. Checking Archives may also greatly change the scan size since all normal archives are excluded otherwise.
Thorough + Archives may take significantly longer to scan though…
I didn’t do a thorough scan that’s why. I wish there was a friendlier way to design the “Settings” in AVAST so scanning is easier. Too much ambiguity.
Try one of the other skins or even try the skinless option. Silhouette_4in1 is good, now you have made the discovery you won’t have that problem next time.
The default settings of Standard sensitivity without Archives is fine for the average user and they don’t have to change anything. The Through sensitivity with Archives takes a considerably longer time.
Having done your first Through scan with Archives you won’t be in a hurry to repeat it. With the various levels of resident protection with standard shield as the fall back a thorough scan IMHO isn’t warranted. The same goes for Archives, which by their nature are inert until unzipped and the executables run, avast by default would either scan these on extraction or before an executable it run, so they aren’t an immediate risk.
I do a weekly manual Standard scan without archives as a part of my weekly maintenance.