Once I downloaded avast! Free, it definitely did not disable Windows Defender (Windows 7). I disabled the Windows Defender real-time protection and the auto scan myself until I found out if it was okay to run both that and avast free.
It does seem to be anti-spyware as opposed to anti-virus.
From Microsoft's website: "Applies to Window 7. Windows Defender is your first line of defense against spyware and other unwanted software."
I don't normally put 100 percent trust in Wikipedia, but..."Windows Defender, formerly known as Microsoft AntiSpyware, is a software product that helps combat malware. Windows Defender was initially an antispyware program; it is included with Windows Vista and Windows 7 and is available as a free download for Windows XP and Windows Server 2003. In Windows 8, however, it is upgraded to an antivirus program."
With the varying opinions, still wondering.
(In case anybody wonders...my signature on these posts shows Windows XP because that's what I'm using on my current computer. The Win7 is the new computer.)