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Windows Defender (an anti-spyware) running in real-time should not conflict with avast! AV, at least on Windows operating systems prior to Win 8.
In Windows 8, Microsoft re-badged its antivirus (Microsoft Security Essentials) as “Windows Defender”, which is confusing. With Win 8 you would not want to run Windows Defender AV in real-time alongside of avast! But with Win 7, both can run in real-time. Likewise, the Windows Defender auto-scan is safe to run in Win 7 and should not conflict with avast.