Actually, I'll argue that VERY old point. I had a horrid virus/spyware problem at my grandmas on her Acer, running Vista Home Premium. It wouldn't even boot without going into safe mode.
I thought I was screwed when it kept stopping scans a quarter of the way through, and even BSOD'ing sometimes, but alas, I couldn't believe what saved me. I was looking for another installed AS, and found "Windows Defender", and decided why not, might as well give it a try. I had used it before, and it missed 3 or 4 things that SAS found, but this time, it came through. It got one of the infected files off and allowed my computer to boot normally again. From there, I installed my good anti-spyware programs (Malwarebytes, etc.) and ran them in safe mode, then downloaded AVAST! and ran a boot time scan, and then proceeded with all other measures to ensure I was clean.
All in all, having Windows Defender built in can save your *** if something similar happened.
PS, this happened while I was changing Anti-Viruses from AVG to AVAST!... so I was unprotected.