Boot Failure

Respected Sir/Madam,
I am using Compaq Presario C796VU laptop. I had installed Windows XP vienna and Windows XP SP2 and was using both nicely. Now, the problem is that none of the OS s booting. When i am trying to boot particular OS, it is taking me to blue screen showing STOP message. When i am trying to install fresh copy of windows XP, i am getting error as "There is no disk in the drive" and no partitions are being shown.Some times i am getting error as "failed to format the selected partition" after proceeding when partitions are shown......... I will be very THANKFUL to you if you solve this problem at the earliest.