Try taking the hard disc out and putting it in another PC running Win2K or XP (but so that it isn't booting off that drive - try unplugging the CD drive and putting the HD in it's place). When Windows starts up, it should detect an error on the HP's hard disc and repair it quite easily...
Put the HPs drive back where it belongs (after shutting the PC down!) and it should be OK. This has happened to me a few times following power cuts...
Chris
June 2005