Thanks for the suggestion. Apparently, it was not the case that anti-malware programs were conflicting, but rather that one (or more) of the files in the i386 subdirectory was infected/damaged. I tried to edit the (apparently) offending file, but as soon as I navigated to the i386 directory, the machine froze up and required a hard boot.
In a word, the repair was to uninstall and re-install SP2. Pain in the neck, but it worked.
FWIW, I think I got infected through the newsgroups.
JET