Without the Win7 installation disc, I don't think there is much you can do.
If you have a DVD that reverts your computer back to the way it was when you purchased it, you could use that.
Call Microsoft and ask them.
If you have the Win7 installation disc, you may be able to install that over Win8 or reformat and install it. If you don't know how to do this, it is best not to attempt it.
You might try taking it to the computer repair shop and see what they can do for you.