Need help!

I got this computer that i think is really good but...... IT SAID I NEEDED AN WINDOWS INSTALATION DISC!!!! me and my mom looked and there like 100 bucks!!! Is there one thats WAY cheaper like in the 30's or some how I can get it free? (P.s. without the disc, I cant do ANYHTHING with it!) Hurry and reply, before i return it! Thanks so much!