Malwarebytes is designed to supplement your existing security software NOT replace it, and I only recommend using the free version.
There are some mentioned under the 3rd Party header about half way down this page: https://community.mcafee.com/docs/DOC-2168
I use Vista SP2 and Windows 7 SP1 and I am using Windows Defender & Malwarebytes free in addition to my McAfee software.
Also always keep your system totally up to date including any browser (IE) updates, whether you use it or not.
I hope that is XP SP3.
No, my new PC is Windows 7 SP1. I'm dumping my old Windows XP SP2 PC as it has developed Windows software issues and I can't update it to SP3 even after performing various "tricks" advised by Microsoft. Anyway it's too old and slow to be of any value even if I wasted my time re-installing everything to fix the issues. The new PC is so much faster.
I'll take your advice and use Malwarebytes for starters. I might install SpyBot later but as I've done on my XP system I'll disable all the real-time features, such as Teatimer.
It's your choice. Just don't install/activate the 'Teatimer' feature in Spybot.