Currently Windows XP, and planning on keeping it with XP only for the forseeable future. Used to have on this machine:
*Windows Vista. Got rid of it because it wouldn't run all the programs XP would
*Ubuntu. Got rid of it because GRUB corrupted my master boot record, which was very inconvenient.
*Windows 3.11 (and thus DOS 6.22 as well). Got rid of it because I couldn't get my Windows 98 upgrade to work, and I was eventually able to get XP. But it did work (natively) on my new hardware, if not ideally.