Start rebuilding a new server and prepare to restore, hopefully the server setup was documented - that will make your life easier

Make sure that your database is secured

Backup and restore

Remove / rename your tempdb files and let sql re-create the files.

Cheers

I had a similar problem some time back and discoverd that I (domain user) owned the jobs and i had changed my password so I changed the jobs to run...

It sounds like you have hit the maximum job history log size - just increase or clear and that will solve your problems