Windows 7 - System not displaying the correct date & time

Hi All,
I am using Windows 7 32 bit. Recently i am facing a strange problem with the system date & time (system clock).
My system is not displaying the correct date & time. I need to manually adjust it.
The time and date of my computer remains the same as that of the time and date that I shut down my computer, and it continues from that time and date only.
For example, if I shut down my computer on 25 Jan 2010, 11pm, even when I switch on my computer on 28 Jan 2010, 9pm, the date and time shows 25 Jan 2010, 11pm

Plz suggest me how to get rid of this problem without reinstalling the OS again.

Thanks in advance,
Troy.

Desktop or Laptop PC ? Have you checked the date in the Bios setup ?

Failing CMOS Battery ?

Mine is a Desktop.
Even the date in BIOS also reads the past date.
May be, i need to replace the battery.