I agree with Downside that PCs don't normally slow up for nothing
Rather than Defender, you could try the trial version of Prevx1....if you have picked up something nasty in your PC, it will find it and stop it from running, plus it will get rid of it.
I'm not sure about Defender....so far it hasn't caught me anything, whereas Prevx1 and Avast have both dug out deep rooted nasties.
Al.