I've got a homebuilt computer running XP pro that is constantly losing time. Over the course of a day the computer's clock will be off by 15-30 minutes. I've got the computer set to use an internet time server to update itself. When I force an update from time.windows.com or NIST the time corrects but within an hour or two it's off by a minute or so. Another forced update repairs the problem.
Oh, and this is all while the computer is running so I don't think it's a bad battery though I'm not ruling it out. the motherboard and battery are both about 6 months old.