I assume you have the files for a CD/DVD. Put them on a USB drive.
You can make a USB drive bootable (google it, there are lots of web sites with "how to's".)
Then just boot to the USB drive in the same way you would boot to a CD
Ok I think i will buy a cd drive and a system builder dvd because i found out that cd drive costs only 20 bucks, and also i found out that windows store doesnt even sell win7 just win8 :/ or maybe i download win 7 for free from piratebay, do you think that its safe? and win 8 is worse and expensive, why it is worse because it looks like a fcking tablet! And you cant get free apps(cracked) for win 8 easily.