window 7

You can buy a single OS and install it to alll other computers but don't you think you are doing illegal and non profit to Microsoft. So, what i would recommend is to buy a Single OS ie windows 7 and buy five licenses. This would prove cheaper. If windows 7 HP costs 6000 Rs INR. You will get a single licese less than that. It is a great profit to you.

it didnt work to my acer 4736z

No you can't because the OS Disc that you buy is licensed for only one computer if you use it on multiple computers you are breaking the license agreement and would have to destroy the Disc and all materials with it as you can't use the product for multiple computers unless you pay for each and every copy of them so each computer has to have its own license key. If you don't have the $$$ to do this than your only option would be an Open Source Operating system such as Ubuntu or Fedora. Which depending on what you use your computer for may be an okay choice for you. They are also free as in price and that you can modify them give the disc to a friend or whatever you want to do under the terms of GPL version 3.