Second hand gaming rig build challenge!

Hi

I am trying to build a gaming rig from second hand components looking at £200-£250 budget. Suggestions please.

Thanks

Is there anything you have already or do you need the full works

And also do you need: monitor, OS etc

monitor not needed just the box.
OS is needed but xp or vista is acceptable.

Going by current 2nd hand prices that's going to be very steep esp inc OS