I am sorry I posted my query to the wrong thread, but I had difficulty trying to find out how to start a new thread. Thanks for the replies. As I was also having trouble with corruption occurring in an audio project I was creating, I decided to completely remove NXT2 and start again. I used the Windows program control panel to remove both the application and the 'contents'. I then rebooted the computer and also cleaned out the registry using two programs (MaxReg cleaner and Trend clean-upfreeze.pdf). Then, having switched off the virus software, I reinstalled the two packages. Unfortunately, I still get the same problem when I try to export the video. This happens whether I am exporting a pure video I have created in Videowave, or using the Slide show app to create a video from a few jpg files.
My computer is fully updated with all the latest updates from Windows and Roxio (none required).
I have also tried going into the tools/options menu and changing the rendering from Hardware to Software and unchecking the Intel Enhanced box. When I change the rendering to Software, I don't get the render graph error, but the rendering freezes at 99% and never creates an output file. Then, when I change it back to Hardware rendering, I don't get the render graph error but it still freezes at 99% (see attached file)