First thing I would do is post this in the Navisworks forum.
The second thing I would do is check my memory. Are you running a 64 or 32 bit machine? If it is a 32 bit machine then make sure that you close down all programs but Navisworks and try to open it up again. If it still won't open then you probably need to upgrade to a 64bit machine with 8gb of ram minimum.
If you are already running a 64bit machine and still get the error then check how much memory you have. Again you should probably have 8gb minimum depending on the size of your models.
You can also check your processes and how much memory each is using to possibly find a program that is hogging up alot of memory.
Rebooting your machine at least twice a day will help. When I had a slower machine I always rebooted in the morning and at lunch time no matter what.
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