This may have been covered elsewhere, but I took a look and didn't see it mentioned.
If you are using MS Office and ask Norton to scan office files while opening them, it will cause Microsoft Project to "stop working" in Vista parlance. Vista eventually issues a message saying that the file you're trying to open can't be found. I'm running Vista Home 64 bit with all updates for both it and Norton products installed. Anti-virus works fine with other office files, like Excel and Word, so it seems to affect only MS Project files. It took me a while to figure this one out as I was installing a trial version of MS Project and thought it was at fault. The light bulb finally went on when I noticed the "Requesting Virus Scan" at the bottom left of the Project screen just before it crashes. Sure enough, when I disabled this feature, Project works just fine.
You can enable or disable this feature by clicking on Norton Protection Center Options, selecting Internet Security from the drop down list, then clicking on Real-Time Protection on the General Options screen.