tell us the name of the threat e.g. Threat name: Spyware.007Spy so we can find out why it failed to detect before.
NAV will detect only Spyware and Adware but it will not remove them (This true for standalone NAV or earlier editions of NIS/NAV and Symantec noted it in their site. Not sure on NIS edition with Norton Antispyware). But they provide instructions on how to remove it.
NAV is an antivirus program, not antispyware. If it detects spyware or adware, it is so kind of them to add spyware/adware detections with their antivirus program. I salute antivirus and antitrojan vendors that adds detections for spyware and adware though their product is not offered as antispyware.
"This is not a trivial question,if Norton, Housecall,Spybot, Hijackthis and Adaware are unable to detect an infection for that long how can I be certain that there are no other undetected infections?"
For as long as the malware authors don't stop. We can't rely our own system on protections offered by any security programs alone. They are tools only. What we need is practise safehex, use alternative if the program is unpatched, lock down the browser and the system and if possible do not use administrator account or follow this: Browsing the Web and Reading E-mail Safely as an Administrator
This is why it is recommended to keep a real time protection enabled. We'll never know which one detects or not.
BTW, you mentioned HijackThis. HijackThis should show you the entry of webdir.dll. It's not a detection tool. It's a diagnostic tool.