you aren't by any chance running a dell with windows xp on it, are you?
if so, there's a patch on the dell support site for you...
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Release Title: System Utilities: Dell Time Keeping Utility, Patch, Windows XP, Multi Language, Multi System, A01
Release Date: 3/19/2002
Description: Dell systems with factory installed Microsoft Windows XP Pro or Home may lose time. Time loss rate has been reported as high as a 20 minute loss per hour.
Important Information
The computer user should be logged on to the system with an account that is a Computer Administrator.
Filename: Time.exe
Download Size: 31 KB
Format: Hard-Drive
Format Description: This file format consists of an archive of files that may be decompressed to a directory on the hard drive. The installation can then be done from that directory.
Additional Information:
Fix for Windows XP system clock losing time.
This file is compatible with the following systems running Windows XP:
Dimension 2100
Dimension 2200
Dimension 4200
Dimension 4300
Dimension 4300S
Dimension 4400
Dimension 8200
Dimension C
OptiPlex GC
OptiPlex GX100
OptiPlex GX110
OptiPlex GX115
OptiPlex GX150
OptiPlex GX200
OptiPlex GX240
OptiPlex GX400
OptiPlex GX50
Precision 220
Precision 330
Precision 340
Precision 420
Precision 530
Precision 620