I have a HP hybrid with an Intel Pentium PU T3200(2ghz, 2mb ram). It has failed to boot up as usual. Had been very slow beforehand and now when I enter set up it does not recognise the hard disc SATA0 and keeps trying to boot from the CD/DVD drive. Before I make a bootable CD is there any use in enabling the 'onboard LAN Boot ROM". Can this be used to start windows xp?
The LAN boot ROM won't help you get Windows started - it's used to boot a machine over a network by loading the OS from a server. I use it when I have to install/reinstall Windows to roomfuls of machines (see "re-image").
It sounds like your hard drive has died - the slowness you were experiencing was probably caused by it having to retry reading data many times. It's worth checking the cables just in case it is simply a matter of the plug falling out and the drive is fine after all but most likely, you'll need to replace the drive and reinstall Windows from your recovery discs.
LAN BootROM does not have to do with network booting.
LAN BootROM has to do with providing gigabit functionality on older operating systems that don't have proper diver support, such as MS-DOS. If your operating system was made after 1999, you should probably leave it disabled, as that will allow your machine to boot faster.