finally found my problem. since it took me hours of detective work, i'll share.
every scrap of research i could find indicated that the problem was probably a keylogger. i tried several freeware keylogger dection programs, but couldn't find the culprit. then i stumbled on a nifty piece of shareware called Security Task Manager.
Using this program, I identified a generic svchost.exe process. now i had also found this process by using Process Explorer, but there wasn't anything really helpful in the Process Explorer description. But Security Task Manager actually extracted a long list of text strings from the .exe file, which clearly identified the process as "007 Spy Software" by Spy Software Inc.
Security Task Manager also allowed me to kill the process, which allowed me to install Spybot S&D, which found the rest of the components of the software and successfully deleted it.