Hi Muhammad,
I cannot tell you if you will be able to give the exam on a Windows 7 PC. I can only point you to Pearson VUE system requirements ( home.pearsonvue.com/pmi/onvue ), where it is clearly stated that "Windows 7 is strictly prohibited for exam delivery", so I would not take the risk.
Did you also installed the OnVUE application successfully? My expectation would be that this application would not run properly if at all compatible on that OS.
I highly recommend you to check with Pearson VUE directly, by contacting them via chat or e-mail, so they can better guide you through the process.
As for the PrepCast scores, there cannot be a predictor of whether you will pass or fail the real exam. What I noticed was that I was slower on the real exam than I had performed with the mock exams - I guess because I felt quite a bit anxious that day.
During my preparation, I noticed that - if I spent too much time on a question, I ended up overthinking, without much improvement in terms of getting the right answer. What helped me, instead, was to mark that question, and then go back to it later; surprisingly, I was better able to identify some keywords in the scenario description that led me to answer that question correctly.
Hope this helps! Good luck on your exam.
Gabriella