For the benefit of others who might be wondering (or who might also have this problem), I'm pasting my response to you from the support email:
We haven't seen an error like this and without more specific steps to reproduce it, we can't really troubleshoot it. We do have a number of users on Windows 7 so I don't think it's a fundamental bug in Terragen, rather it must be something unique in your configuration.
Perhaps it is a permissions issue. Do you normally run under a limited user account? You could try running Terragen as an administrator or even uninstalling and reinstalling as administrator. Also make sure you're writing to an area that you actually have access to under the current user account (for example you cannot save any files to the Terragen install directory by default under Windows 7).
If none of that helps, provide as many additional details as you can regarding error messages, specific save path, etc. and we'll see if we can help you resolve it.
- Oshyan
