Hey what if you install windows now and want to pay later after you are fully satisfied with windows performance.....
Here is how can you extend Trial period of windows to 120 days.

Step 1 : Go to command prompt. Run command prompt as administrator.
Step 2 : Now type the following command and press Enter:
slmgr -rearm
Step 3 : Restart your system and you will get your trial period extended for 30
days more.

And you can run this command for 3 times. that means (30+ 3*30)=120 days
Remember that you run the command at the last day as running the command will discard the days left in previous month.

Although its not a very good practice but you can use this to check if windows serves you what you need.