Manual Harddisc selection necessary at Boot

I had an NTFS partition until recently, but it worked fine after formating and reuniting with the Mac Partition.
The issue came up after trying to make the Huawei 3G USB Modem work.
Anyway, the problem description is the following:
after rebooting or generally powering up I get a black screen stating roughly the following :
"Insert a bootable device and press any key"
Unfortunatelly pressing space, or any other key, did not solve the issue.
Hard-shutdown, option-key, Macintosh HD (for lack of creativity) and it boots fine.
My troubleshooting consisted of using disk utility, yet "repair disk" remained fruitless.
Suggestions are highly appreciated,
Thanks,
Toma

System Preferences

My mum woke me up this morning cause the internet sharing wasn't working,
when I got there I saw she had already opened up system preferences. That's when I saw Startup Disk for the first time 🙂
Note:
She never used a Mac, she just spent 1000$ for a Vista PC without consulting me first. Too bad, a Mac Mini would have been great.
Happy new Year