If you are talking about Windows Defender.

I tried it and it brought my PC to a crawl. After I deleted it my PC was working at more than twice the speed.

BTW I'm using Panda Platinum Internet Security and allthough annoying at times (having to press "always allow this connection" like 10 times before it figuring out that this connection should always be allowed) as a whole I'm quite happy with it. + It does not use as much CPU as some others on the market right now and it finds pretty much anything annoying.

There is one problem with it though. It keeps deleting the EviD Patch like once a week. I then have to turn it off and reinstall the patch and I'm good for another week. (for some reason it won't be added to the exeptions list)