my system is dell optiplex GX270, 2.80GHz.
In the my computer properties it shows 2.80 GHz after that it shows 1.40 GHz in the second line.
But when i enters the system setup through F2 it is LABELED at the top with dell optiplex GX270 series Intel Pentium4 Processor: 1.40 GHz Bios version A07.
Again in the CPU information it shows clock speed 1.40 GHz.

somebody tolds me to update the BIOS i have updated it from A06 to A07, but the problem is still there.

If anybody knows please help me out of this problem.
thanksI'm not entirely sure, but I think I KNOW why.

The processor must be a dual core running at 2.8GHz. This means that the TWO cores are individually CLOCKED at 1.4GHz.
Thus, they both work to a clock speed of 2.8GHz.Hi Carbon dudeoxide
Thanks for your suggestion,
but
the actual cause was not that;

The only reason was the processor's bus speed which was 800 MHz.
on the other hand
system board supports only up to 533 MHz only.
By changing the processor not only the BIOS setup & clock is showing the right speed but also the speed of my system doubles.