I agree,
Any software that does virus/malware detection will hamper system performance in one way or another, best things to do are:
-Never ever ever use IE unless you trust the site you're going to (Like netflix on demand), always use FireFox (install adblocker if you want, free)
-Never check email on the HTPC using a local client, ESPECIALLY not microsoft clients, use web based email only and never open attachments.
-When you install software do so carefully and sparingly, ask yourself 'Do I really need this software on this machine?' and only do so if you trust the company.
A little prevention is worth way way more then dealing with antivirus software.