I would be willing to bet that you have "auto update date and time" checked in your settings This will auto update that info every 24 hours to Eastern Standard Time. I had my computer do that to me....I live in Germany so everyday my computer was 6 hours off. With this option checked....Windows will syncronize your computer time every day. That is great if you live on the East Coast. I use Vista and this is how I fixed that exact same problem
1. Go into control panel
2. Select date/time/region settings
3. There will be a box that is checked with the "auto update" option
4. Uncheck that box
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Case : XION II w/12” green cold cathode
Motherboard : Asus P5N-E 650i @ 1532 FSB
CPU : Intel Q9300 @ 2.875 GHz
CPU Cooler : Cooler Master Hyper TX2
RAM : Corsair XMS2 Pro/pc6400/ 4x1GB/ @800 MHz
Graphics : Evga 9800 GTX @ 760 MHz
PSU : Antec Neo Power 650w / modular cables
OP: Vista Home Pre 64 bit