Windows Games Mysteriously Halting

My son found a problem with his computer. Sometimes, the game he was playing would halt after it started. Then he said that if he defragmented the hard disk, the game would start running again. I didn't think too much about it until the problem occurred yesterday evening. When I examined his computer, I realised that it had run out of disk space. The game didn't crash; it was waiting for some spare disk space to write its files. When he defragmented the hard disk, some space was freed up, so the game could continue running. I freed up about a third of the hard disk and there's no problems running games again.