This could be several problems but we will
assume a dead cmos battery.
If the cmos battery is dead then the settings
for the harddrive have probably been lost.
Right after you turn it on check to see what it
says about going into setup(cmos). It will usually
say press Del or F1 to enter setup. If it does
then press the DEL key or whatevery it says.
If you get into setup then you need to look for
the option that says something like AUTO DETECT
HARDDRIVE or something similiar. If it has that
option use it to detect the drive then save the
changes and reboot and see if it will go into windows.
If it works you will need to either replace the
cmos battery after you turn it off or go through
this procedure each time you turn off and turn
on the computer.