The fact that NAV will not open cofirms it is a virus.  Many of the newer viruses disable the anti-virus programs.  You will need to disable system restore to remove all traces of the virus.  Instructions disabling/enabling are here.  Once you have disabled system restore, run the online scan and remove all viruses.  Then enable system restore again.  Update your anti-virus immediately after this.  Also are you visiting the windows update site and installing the critical updates?  Many of these updates will patch holes that are allowing some of the new viruses in.

You most likely have a virus, Will your anti-virus run?  Has it been updated?  Run the on-line Housecall Scan from Trend-Micro to see if you can identify and remove it.  It is available here.  You will most likely have to disable system restore if a virus is located, scan and remove the virus, then re-enable system restore. 

After you have tried the good advise by YellowHammer, If it is still a problem try this :-
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Failing those solving your problems a post of a hijackthis log for the experts to advise.
HijackThis From Here
Download, run, scan, save log, then in notepad copy the FULL log by copy and paste to a new message in http://forums.us.dell.com/supportforums/board?board.id=si_virus

And an expert with HijackThis Knowldge, will have a go at giving advice. Please note the list of experts names below, very few forum regulars here have had this training.

DO NOT FIX ANYTHING WITH HIJACKTHIS WITHOUT EXPERT ADVICE, most of what it finds you need for normal MS Windows tasks.

Known Spyware HijackThis fighters in DellTalk - If you are, and are not on the list please PM Me.
ChrisRLG
therock247uk
TomCoyote (of http://tomcoyote.org/forums/index.php fame above)
irelynmisses
YoKenny
baskar1234

You could also go to one of the more specalist forums where more experts will be able to help.
http://tomcoyote.org/forums/index.php
http://forums.spywareinfo.com/index.php
http://www.net-integration.net/cgi-bin/forum/ikonboard.cgi
http://boards.cexx.org/index.php
Do read the sites FAQ before posting, and advise your problem and what steps you have already done to try to cure your problem.

I, and the other hijack experts mentioned above, are in all those sites (and more) with the same login names. You might get one of us at those sites also to anwser your log, but other experts will also be available.