Intermittent Inaccessible Hard drive

Hi all,
I have WinXP Pro running on a laptop and since yesterday it is intermittently giving error on boot stating inaccessible hard drive, sometimes it boots up slowly and crashes again with the memory dump blue screen. I have very imp data which I have tried backing up during the brief “up times” with no luck. I have tried using ERD Commander but it is of no help as it cant find the hard disk to begin with. I suspect boot sector virus but am not sure. I need the data off the hard disk. Please help