Does this occur with any project file, or is it a specific one? Have you tried to open it on another computer?
If the error is tied to your project file (and not to the computer), then please post it here and I will try to reproduce the error and investigate. Otherwise, it sounds like some of your NI software has become corrupt somehow, and your best best is probably just to completely uninstall and reinstall all National Instruments software on your computer.
Eric V
National Instruments
Applications Engineer
Certified LabVIEW Associate Developer
"I'm a Ramblin' Wreck from Georgia Tech and a helluva (NI Applications) Engineer!"