Quick Fix: have a program named procmon.exe running (copy of notepad.exe) to disable malware temporarily. This should let you run searches & download fixes. This is only temporary while you clean the system. Read this post for more details and please let me know in comments if this does or does not work for you.
The system did indeed have a corrupted atapi.sys file as noted in the comments, though I did not end up using ComboFix to clean it – I was able to replace the file with the identically-sized but binary-different one from the most recent service pack (C:\Windows\ServicePackFiles\i386\atapi.sys) and have not seen the same problem recurring.
In addition, if you need to prevent it from redirecting while you download fixes, you may be able to simply copy notepad.exe (or another common executable) to the name procmon.exe and run that. Last night while I still had the infection active it did not seem to redirect while procmon was running under that name, possibly as a measure to avoid detection.
At this point (several days after initial infection, more than a week after initial reports in the wild) I suspect that many of the tools linked below are updated with definitions that cover this malware so you can probably get by with simply running updated versions of those.