Just a test, but this impressed me...
I tried running a full scan in Immunet and MS Security Essentials, simultaneously, while watching a video on Youtube. It did take some minor tweaking of Process Tamer's settings to make the video play smoothly, but it worked.
Then I tried watching another video, with *three* virus/malware/crud scans going on [in Immunet, Malwarebytes, and Microsoft Security Essentials]. Full scans, the whole drive, and any removable drives. Playback was still okay, when there was incoming data, but the network connection got pinched a bit -- probably due to a busy CPU and I/O circus going on in the background. Switching apps became sluggish, but once switched, whatever was foremost became responsive and behaved sensibly.
So. Process Tamer is a pretty nifty little app. It's not quite Linux-level multitasking, but it does keep the bastards in line.
In case you're wondering why Youtube, it's because Flash is very inefficient, uses more CPU cycles, and probably more memory for playback than a comparable .avi movie and player. And streaming media requires a smooth-flowing network connection. So if anything goes wrong, it'll show up in the form of choppy video, bad audio, etc.
Oh, and I'll only keep one antivirus, just haven't decided which one yet. But still. Wow.