Thanks for your reply.
I did find this option and put it on but no luck. The task was still not visible.
Yesterday evening I did some intensive searching and one of the things I came across was this page:
I followed those steps and at last I was able to see this task in Scheduled Tasks as well as under Windows Explorer. Attributes of the file were still the same however so Archive System and Hidden still on Owwi.job.
I looked in the file with (the basic) notepad and found that it was most likely some sort of left over from a virus I was once infected with. But this file itself was harmless although it was still executed at boot time.
To summerize; I renamed the file and rebooted but no effect what so ever. Now I was convinced there was indeed no risk in deleting it so I did and rebooted again.
File is now deleted, no side effects noticed.