Gaming while defragging
With the automatic defraggers, which use CPU load and disk I/O sensing are usually less resource-intensive than the older scheduled-run defraggers, it should be safe to play while defrag runs in the background. I use a third party defrag utility on my XP Pro desktop, and I play COD4, Mass Effect etc, while it's defragging some drive, but it has never affected the FPS. Very transparent.
i2500, Asus P67, 8GB, 4TB, GTX570 OCed, Xonar DG, Logitech G15, Razer Death Adder, Dell 2407WFP-HC.