My system (and drive) are rather antique ... the physical drive is 40G of which 35 is the main (C:) partition and the rest is an "invisible" partition used by Restore-IT for its quasi-imaging function. Typically I've got about 70 percent free space, give or take 2 or 3 points, so 40G is fat for my current usage.
Anyway, back to topic -- on my drive, avast takes up 181 MB in Program Files and another 12 MB in Docs & Settings. (Probably also some drivers go into the system folders, but I didn't check there.) So even if I totally removed avast (and didn't replace it with something else), I wouldn't gain as much as 1 percent free space even on my small drive.