Hard disk is read only (by 2 users)

i was using samsung 40 GB hard disk for 3 years. now it has become as if read only disk.
suppose i delete some files on it when i restart it returns back to same location.
if i copy any thing on hard disk and restart that thing is lost.
i tried to format it,but could not because error message is shown "cannot format"
if i try to install new os in between of it my pc restarts and same old problem ocurrs