|It's not booting from the hard drive. Either the drive is:|
3) Has a bad connection
4) Disabled in the boot order
5) Disabled in cmos/bios setup
For 4 and 5 go into cmos/bios setup and verify it's in the boot order and is enabled as a boot device. If there are options for both SATA and IDE/ATA you need to make sure you've got the right one specified. You may need to consult the manual for that. If the bios is seeing the drive it'll show on the posting screen when you start it up and probably in cmos. If the HP is showing the full screen logo I think you either hit TAB or ESC to show the posting screen. You can probably enable the diagnostic (posting) screen in cmos/bios setup also. If the drive isn't being seen at all then that's where the problem is.
If it's not seeing the drive, open the case and make sure the drive's data cable is in good condition and tightly connected on both ends. Also check the power connector. If you start it up you may be able to tell if the drive is spinning up by putting your hand on it to see if you feel any vibration. If it's not spinning up, or is spinning but not being seen, the drive is probably bad.
If the drive is being seen you should probably download a diagnostic from the drive manufacturer. Generally it'll be a cd image file that will allow you to create a bootable cd and from there you'd run the diagnostics. If it passes then maybe you just need to reinstall windows.
Real men don't use AntiVirus; they just reformat