Its urgent

hi folks..
I am using windows XP professional machine and i have connected it my
domain. But the time always becomes late by 1/2 hour or 1 hour..

i bought 30 hp pc,s(directly from HP) only two months before for my
clent location. So i am getting confused why the problem has occurs.
Some told me if it is connected to Lan it took the server time.

But even if i disconnect it the same problem has occured. this is very
new pc so i think no need to change CMOS Battery . I tried all possible
ways (resetting the time by disconnecting lan(Updating new BIOS
Version)).

so help me because this is very urgent