Hi there. You obviously did something wrong as installing 3rd party aircraft should not affect your other aircraft in any way.
It's difficult to troubleshoot problems at long range without specific details. I think the quickest way to fix this would be to Uninstall/Install FS2002. Then read this installation tute & try again.
http://www.simviation.com/acinstall.htmPS. If gauges are included with a download paste the individual GAU files into FS2002\Gauges. Do NOT overwrite existing files if prompted.