I think NAV is a red herring: after all you have been running it long enough for the subscription to run out. Once the subscription runs out you only lose the ability to update the virus definitions. Did it suddenly stop booting or did you do something just prior to it misbehaving? Safe mode is the best way of uninstalling rogue software but if you cant get this then the next step is to use the recovery console which entails booting up from the XP CD. More info here:
http://www.wown.com/j_helmig/wxprcons.htm