Windows Defender VS Spybot

You would need to be alot more specific on what is really being found and on the exact setup and configuration of your programs including whether Ad-Aware SE is the Pro or Persoanl (free) version. Also which scanners were run first in what order. In reality we find Ad-Aware not to be that useful at detecting real malware problems. It spends too much time telling you about Cookies and MRUs which are a waste of time. I seriously doubt that Spybot truly finds NOTHING. Just like Ad-Aware it will tell you about cookies at a minimum.

Do you need Spybot? Yes, you need to configure the Immunize feature and use SDhelper. They work well and require no realtime resources. Having Spybot around for a secondary scan is also useful.

Windows Defender is a totally different story. Microsoft has taken the once very useful GiantAntispyware program and turned it into a tool that is not that useful anymore. Does it provide some useful features.....yes!
Does it block some malware and protect various settings....yes!
Is it better than nothing....Yes!
Is it better than a paid commerical package that provide realtime protection.....absolutely NOT!

The main reason you would keep Windows Defender is to have a realtime blocking tool. If you are willing to buy one, then I would definitely get rid of Windows Defender and use the one you buy (like Spy Sweeper, Spyware Doctor, AVG Anti-Spyware, CounterSpy for a few examples). If you don't want to buy something, there are other free alternatives like:

• Spybot's Teatimer - which we don't really recommend but some people like it.
• SpyWare Guard - also not that highly recommended since it is so out of date with the current state of malware
• Spyware Terminator

You should also use SpyWare Blaster which also does not require any resources and does not need to remain running to provide protection that it offers.