HijackThis does have certain issues in displaying certain files. So in some occasional cases, the "file missing" attribute is an error display, whilst generally it is genuine. It is usually upto experience to determine if it is genuine or not. Let's not argue about this.
For O23 entries, often times they remain unremovable by HJT even when the files are gone as O23 entries refer to services running on the system, and HJT does a poor job of removing services. The best way is to manually disable the service.
Even then, some cases are clear cut HJT errors as the files are legit system files, for eg,
O23 - Service: @keyiso.dll,-100 (KeyIso) - Unknown owner - C:\Windows\system32\lsass.exe (file missing)
Most of the time, these files are not causing any problems to the system.
tleslie: I would suggest posting a screenshot so everyone can have a clear idea of what is exactly happening. It doesnt seem to be a malware problem, but a screenshot would clarify things just in case.