Windows Defender With Norton or McAFee ?

I disagree with running WD and Norton 2010 together. Norton 2010 includes an anti-spyware component. WD is an anti-spyware program. You don't need two real-time AS programs running at once. Not just because it is overkill, but because now you have two programs watching the same resources for the same type activity. That sounds like having two sets of eyes watching an asset, and so it "sounds" like a good thing. But perhaps it is best to look at it as two guard dogs watching the same bone. Now it is not such a good thing as the dogs may have conflicts, wondering what that other dog is doing eyeballing his bone.

So I recommend keeping one AS active in real-time, then use the other for periodic supplemental manual scanning.

I'm using Kaspersky Internet Security 2010 which looks for spyware and other harmful files. I also looked to see if Windows Defender is running with Kaspersky installed and it is. I haven't had any problems or conflicts between these two products. Windows Update is always downloading definition files for Windows Defender so I know that it's working and doing it's job...hopefully!

Yeah, they may play well together, but they may not - and that is the danger. But a complete overlap is just not needed either - assuming you keep your computers and security solutions fully updated and you don't intentionally participate in risky practices, like visiting illegal porn and gambling sites, or partake in illegal filesharing at P2P sites.