ADDITIONAL INFORMATION: Internet Explorer, almost never opened, has not done this. Process Explorer indicates that one instance of svchost.exe takes up an inordinate amount of memory-- I have not been able to tell exactly what service is responsible, but I can End Task on it, with no obvious consequence for browsing (no service is obviously masquerading as legitimate, as I have seen before). AVG Anti-Virus has indicated that it has found and quarantined at least one trojan, so presumably they are not in effect. Finally, Browser Hijack Retaliator periodically indicates (on the Browser Extension tab of its IE Plus page) that an extra toolbar is operative with a CLSID of DFB852A3-47F8-48C4-A200-58CAB36FD2A2, a search on which did turn up potential clues. I delete the toolbar when I find it, as no other details are provided. I notice no unintended consequences when I do so. Finally, and probably (?) unrelated, my sound capabilities through Firefox keep disappearing ("no sound device") until I reboot and they are (temporarily) restored.
STEPS TAKEN: I have installed, updated, and run the following anti-malware programs, to no apparent effect: Spybot S&D, Ad-Aware, PrevX, Hijack This, Browser Hijack Retaliator, AVG Anti-Virus. They remain installed for further logging and testing at your request.