how to fix on "unexpected error ..." msg, plz help

I have a serious problem with my NOF 12 running on Windows 7. I tried to find a solution on the board but could not locate any information.
I get an error saying “an unexpected error occurred writing this file. In order to preserve the integrity …”
I have tried several things that I have found on the internet like delete pages that might be the culprit. I actually deleted about 25 percent of the over 800 pages we have on the site, that didn’t work.
I tried to export the template and base a new site on the template but nothing, it would crash
I also tried to reoptimize but same problem.
I have several backups but they have been damaged also.
I did purchase the latest NOF but I haven’t installed it yet due to this ongoing problem.
Any suggestions? This is a corporate information site which took a long time to build and it needs to be up and running again. I can rebuild the site but will take months if not a year and we don’t have that time.