missing os

HI MY NAME IS RAJESH FROM INDIA. I AM FACING A PROBLEM IN THE HDDRIVE .

WHENEVER I INSTALL WIN2K (OR) WIN XP AFTER FINISING THE FILE COPYING. WHEN MACHINE RESTARTS

IT HAS TO BOOT FROM HDD BUT IT SAYS MISSING OS. IN THIS SENARIO I TRY TO INSTALL WIN98 IT IS SHOWING

5891MB TOTAL DISK SPACE.BASICALLY HDD WAS 40 GB BARRACUDA ATA. WHEN I AM BOOTING FROM WIN2K CD

IT SHOWS 40GB. SIR WHAT COULD BE THE PROBLEM PLS KINDLY INFOM ME..THANKING YOU WAITING FOR U R REPLY.