I figure there are two possiblities. LabVIEW is poorly installed on your system causing the error or your VIs are corrupted. You can test the former by installing LabVIEW on another computer. If you can open the VIs there, consider contacting NI for ways to more throughly remove NI products from your machine. To test the latter, see if you can create new VIs in 7, save, close, and reopen them. You can also see if other VIs on your machine open. If your VIs are corrupted, then try to open them in your older LV, copy the Block Diagram over to a new VI, save, and open in LV 7.0. At the very least, you can get a screen shot of the diagram.
Good luck!!