It is not a rounding problem. The error gets increasingly longer at a rate of several seconds per hour.

This is on Windows 98. I don't see how it could be a granularity issue. The problem is that the error keeps accumulating.

Using your code, I was able to preproduce the problem:

Core : 1081092725
HiRes: 1081092725.097

Core : 1081092785
HiRes: 1081092785.098

Core : 1081092849
HiRes: 1081092845.099

Core : 1081092908
HiRes: 1081092905.1

Core : 1081092968
HiRes: 1081092965.105

Core : 1081093029
HiRes: 1081093025.106

Core : 1081093088
HiRes: 1081093085.162
[download]

Here it shows a discrete jump of about 4 seconds. No idea why.