Windows Boot Manager / system not dual boot

Hi there, first time posting. I have a problem on my system that on startup it opens windows boot manager and I have to choose to open windows vista. The only other option is to do a diagnostic scan, (which I have tried selecting with no results.) From What I have read, I understand this page would show up if I had multiple operating systems, but I don't. It didn't do this before. Can anybody help with what may be the issue?
Thanks in advance.