I've tried Spy Doctor; it found several "infections," including three Trojans. I deleted them and both Spy Doctor and PCCillin now say the machine is clean, but the problem persists.
I checked the Addons in the Manager and disabled several I didn't recognize. I found that one -- HIWUMEKU.DLL -- would not stay on the disabled list. I ran a Google search that seemed to indicate that HIWUMEKU is a recently used Malware name, so I suspect it might be the culprit. However, it won't stay disabled and I can't figure out how to delete it (there's no DELETE button in my Addon Manager dialog box).
I've attached the Hijack This log and some screenshots of the offending windows. Would greatly appreciate any help you can give me. Thanks.