Dell system (Dimension 3000, XP SP3) recently updated to IE 8.  I'm trying to figure out first, if Windows Defender is part of or separate from Windows Security Center.  I note that I have a Windows Defender folder listed in my Programs files, but when I open Windows Security System (from Start Page) I don't see the features of the Defender portion.  How do I get the Defender features activated?

Second, I'm running McAfee as my primary security system and which is confirmed by Windows Security Center.  I also maintain "on the side" the free/public Malwarebytes-antispyware security for backup system security scanning checks.  But I'm wondering about dropping the McAfee subscription and going fully with the Windows Security Center - any critiques would be appreciated.