I see it's a 578. Use a System 7.5 disk tools floppy, Apple should have one on their website. Rebuild your desktop when you start up. Some viruses manifest themselves there (though most of them didn't attack System 7 if I recall...I know CDEF doesn't and I'm pretty sure WDEF doesn't either).

Also, Disinfectant and Agax are both free. They're small enough you can fit them on a disk tools floppy if you get rid of Apple HD SC Setup and Disk First Aid.