I had a laptop on the original windows 8 on it
I would not have wanted.
and only has my friend made linux on it
That he has only good intentions. However, I would not have done without denying it with you and to listen what are your activities on a PC.
how can I delete the windows and get back?
Without knowing exactly how it has implemented the installation, so that partitioning and formatting and present that is not to answer. Remote diagnostics are always difficult, but with such a great lack of information, it would be smooth "data murder".
Get used to you but Linux. Otherwise you can download a CD image your favorite window in Micro Soft.
It depends on whether Linux was installed on another partition
Do you have to reinstall everything