Clock randomly changing on random systems

I have approx. 80 to 90 systems in many different office locations. The PC’s are all Dell Optiplex GX 320, 330, or 360 models. The clock randomly resets itself back exactly 1 hour on random PC’s. The systems are not running through a domain, however they are all connected to the same network. All settings are the same (IE clock sync and Daylight savings time check box is unchecked), except for the service packs, some are SP2 and some SP3 (Windows XP). I have checked the BIOS to see if it has an option for the clock to adjust for daylight savings time so I could disable it, however it does not. Does anybody have any ideas as to why they are doing this.