I’m running Windows Vista with Windows Defender as well. A friend of mine who’s been using Avast for many years said to me that I don’t need to run Windows Defender anymore since I’m now using Avast Home 4 having switched from AVG. Is this true???
There is nothing to stop your continued use of windows defender, though I don’t believe it is as good as some other anti-spyware applications, though it does have resident protection.
MalwareBytes Anti-Malware, On-Demand only in free version http://download.bleepingcomputer.com/malwarebytes/mbam-setup.exe, right click on the link and select Save As or Save File (As depending on your browser), save it to a location where you can find it easily later.