Every time I try to save a macro-enabled workbook I get the error "Your
changes could not be save to [filename.xlsm] because of a sharing violation.
Try saving to a different file."

Which then brings up the msg that says that the I file I'm trying to open is
in a different format than specified by the file extension. and that I should
verify the file is not corrupt yada yada yada. End result, file becomes
corrupt.

How do I stop this happening?

I've read something about a conflict with anti-virus but my version (Trend
Penicillin) has not been flagged as an issue. I do have Office 2003 installed
alongside 2007 but don't want to uninstall unless someone can say this is
definitely the issue.