I've had advice before on technical websites that basically all you need running is a decent firewall and a decent anti-virus program. Anything else is not necessary and if they give you any problems, then don't bother running them because quite often there are runtime conflicts.
You might want to spend time checking out this set of forums:
http://www.webuser.co.uk/forums/ubbthreads.php
and, if necessary, asking this same question there because for technical questions I would trust that site much more than a general one like this.
And in the months since removing protection programs like Windows Defender from start-up I have not suffered any problems (I suspect that's not just down to luck).