I think "runtime error 13 - type mismatch" is a visual basic error; i'd guess it's more likely to be a duff application than a virus. Are there any errors listed in the application log (control panel -> administrative tools -> event viewer -> application log and then looks for errors with a timestamp corresponding to boot-up)?
Also, have a look at what is running at startup: Start -> All Programs -> Startup and also Start -> run -> msconfig and look at the Startup tab.