why did my anti-virus program suddenly decide simm city 3000 was a virus

It's because AVs guess if there is a problem, based on a huge database of patterns. Sometimes they get this wrong and it's called a false positive. As this database is regularly updated, something which passed previously may be flagged up now as the AV might not have been searching for that pattern.
You should be fine, but if your worried, upload the file to virustotal and have a check. Mostly it'looks be a false positive.