And here I was going to recommend that you have your PC drink a glass of water upside down, and then scare it suddenly.
Actually I had similar issues with SpySweeper over a year ago. I tried Version 4.5 on my main PC and it was nice; worked very well and no overly heavy hits. I purchased three licenses - one for each of my main PC, my wife's PC, and my notebook. Then they released V.5 and sent it out as a free update, and Wham! It started using 50% CPU on all PCs and would not let it go.
Webroot support would not even acknowledge the problem, just saying it must be a "bad install" and to uninstall/reinstall. CastleCops was filled with users moaning about it for quite a while. Eventually I just wrote them a letter saying what I thought and uninstalled from all PCs. I won't touch it again.
More recently I had an issue more like yours. If I didn't use the keyboard or mouse for a couple of minutes, when I tried again the mouse would move but all else was frozen solid. Tabs on the taskbar could not be clicked, tabs in Firefox couldn't be clicked. I could not close any programs, nor access the task manager 0or Process Explorer. I could not find any indication of what was doing it. Nothing in Event Viewer, and if I left Process Explorer open and showing on my second monitor it showed nothing out of the ordinary. I had to go back to when I believed it had started - about two weeks - and determine exactly what software I had installed, removed, or updated. One I suspected was Clipboard Help+Spell. I had installed that at the right time and it was always running in my systray when Explorer was freezing - at least I surmise that it was Explorer based on the symptoms. I made the list of installed apps since that date and disabled them one by one. Turned out to be a program called FastCheck, which is a notifier for Fastmail's webmail service. I could enable it and see the freeze soon afterward, and then disable it and see everything free up. I subsequently found several users post about it on their forum. The developer admitted it but his response is to have users make several configuration changes in Windows to alleviate it. I figure that's HIS job, so it's gone!