How to tell a macmini to stop booting to Windows from a macbook pro?
The system stores the boot choice in NVRAM, so you might reset it by powering off and then holding command-option-p-r while you hear the start chimes repeat 2 or 3 times.  At that point, it should choose the first bootable system on the internal drive. IF that doesn't work, you could post the filesystem layout - but a corrupt NVRAM settings file would explain why your choice of Startup Disk wasn't respected.