Unless the infection vector is something that the user downloaded and executed, in which case it's sitting there waiting to be found and executed again.
That's a possibility if you repair an installation, but the repair replaces all the system files. I've only seen a repair fail a couple of times, and both were last week with antivirus2008. If you get the system running and get the rootkits out, any competent virus scanner should be able to find dead virus files.
slightly off topic. Microsoft has discontinued support for Outlook Express and replaced it with Live Mail.
Live Mail stores emails as separate eml files. I suspect this is to make it easier for antivirus products to scan and disinfect them.