The BIOS has a small battery to keep track of time. From searching on Google the 1525 is prone to failures in this area - Search
cmos battery 1525
and see how may people are having problems...
A good link is the following which tells you how to fix it...
http://support.euro.dell.com/support/edocs/systems/ins1525/en/SM/coinbatt.htm#wp1179839
Peccavi, February 2009