Importing Mpeg-4 into sony vegas?

I have a job which is to edit a six hour seminar. It is split into 9 clips. They are in mpeg-4 format. I am a bit new to using sony vegas (I have sony vegas pro 8.0) but have been able to import all my projects into it before without any trouble.

I have been able to edit the first two sections fine but now for some odd reason I can't import the third part, here is the box that comes up when I try to:

Warning: an error occurred while opening one or more files.

The reason for the error could not be determined.

I have tried to import it several times and always says the same thing.

I also can't imort the fourth file or the seventh.

The ones I can't open are significantly larger then the others so I'm worried it might be that. I tried opening it also in windows movie maker (to split it into more parts) and I can't.

I am able to watch all the files on media player, I just can't open them in editors.

This is very important and I need to finish asap so all your help is welcome!

Thank you!