Help buying used computer

How can i make sure a computer in good working condition if it dont have window operation system i dont mine installing one but how can make everything going to work thanks.
Answer #1
Download Hirens boot CD burn it to a CD
http://www.hirensbootcd.org/files/Hirens.BootCD.15.2.zip
or USB
http://www.hirensbootcd.org/usb-booting
use the utilities that come with it to test the system.
If you don’t want to do all the above then just use the Windows Live that comes with it and see if it boots up and runs fine.
Answer #2
You must do a memcheck, run a benchmark of the cpu and the gpu and check if the parameters are near the mean values. Check the temperatures and the SMART values also, this should cover obvious faults but remember that there can be some problems more subtle and difficult to reproduce. When you have the occasion make him go under a heavy load.