Well it depends on what the 'something' does - glancing at my desktop I see I have Malwarebytes, Microsoft Fixit, HiJack This* - afaik they are all on-demand programs so there is no overhead. I don't have any other anti-virus software running.
I think some people are trying to compare apples and oranges to pineapples and pine trees here.
Running more than one *real time scanner* of any kind that inspects files as you move them, open them or execute them, at the same time, is a recipe for disaster.
As regards running Windows Defender and any other real-time anti-malware scanner at the same time: If you're doing it on a typical workstation I'm surprised you can even fully boot it up and log in. It may be that those... people... who are insisting on doing this on a server are only getting away with it because it's a massive tank with multiple multi-core processors and dozens of gigabytes of RAM. On an ordinary computer you would know instantly that it's a bonehead thing to do.
Lots of anti-malware products have the option to set up constant real-time scanning, or not to set it up, when you install it. It is necessary to pay attention to what the install screens tell you. Close attention. (In fact, if you are a computer professional, you should, as a matter of course, NEVER accept the default installation parameters for ANY software until you have at least examined the "advanced" or "custom" options.)
MalwareBytes, for example, can be used as a real-time scanner (where it will likely conflict with any other real-time scanner you have running, and cause serious problems) or only for manual or scheduled scans. You get the choice when you install it.
So having multiple anti-malware products *installed* is not a bad thing; in fact, most experts recommend that you have two or three installed, because none of them catches 100% of everything. But having more than one of them *running* at the same time is always a mistake.
As for my experience: I use free AVG at home because, unlike free avast!, it does not require registration. It does routinely try to trick me into installing its paid version(s), but since I am a computer professional I am not fooled. If it really got unbearably annoying, I'd probably go with free avast! and put up with the registration requirement.
At work we use the paid, centrally-managed version of avast! Endpoint Protection. My only real complaints about it are that 1) the documentation is non-existant and the only way to find out how to do something (anything) with it is to post a query to the user forums; and 2) it has a lot of false positives on websites because it insists on treating situations where website developers don't follow current "best practices" as though they were actual threats. (If you're a web developer, I'm sure it's nice to have a tool to tell you that failure to patch something has created a potential hole. But when end-users keep getting nasty-looking warnings because some developer is using an iFrame even though in the particular case, that iFrame has not been exploited to create an actual threat, then those users will likely start ignoring all of those warnings and just click through them until they stumble on something that really hurts. I've had this argument with the avast! developers, to no avail.)
The server-managed version of avast! is quite cheap per-seat for not-for-profits like ours if you get the three-year licenses; they provide a 50% discount to not-for-profits. The product does not require a true server OS; mine has been running on ordinary Windows 7 Ultimate on a fairly robust (think "gaming"), always-on, workstation for years. And the server-console function to easily let you block access to specified URLs (using wildcard characters if you wish) across your entire network works great.
An amazing thing about avast! is that not once in the nearly 6 years I've used the product has it ever forced somebody to reboot in order to load an update. It just gets the updates and runs them, period. Can you show me any other Windows anti-malware with that record?
By the way, avast! recently purchased AVG. I don't know what that will bode for the future but right now they are managing them as two completely separate products.
(And no disclaimer needed; I don't work for avast! nor am I a reseller of it--or of anything else except my own products.)
Ken Dibble www.stic-cil.org