The best you can do is install Win 7 on your other PC at not validate it. You will be able to use it fully for 30 days "trial period" after which you will probably get constant reminders to validate the copy and it will probably shut down every two hours of usage.
There is an exploit to reactivate for another 30 days which you can find by Googling. You can use the exploit three times before Win 7 will demand a activation key; that's a total of 120 day max free trial period. This particular exploit existed since Win Vista. This will allow you some leeway if you need time to save up more money for another licensed copy of Win 7.http://www.howtogeek.com/howto/windows-7/extend-the-windows-7-trial-from-30-to-120-days/