Also try re-seating the RAM. Ensure power cord and monitor signal cable are disconnected. Then simply pop the memory chips up abit and push them back in until they reseat.
Good time to reseat all power and signal cables as well (includng the disk drive and ribbob cables), and ensure the interior is dust free.
I have seen at work, when an HP unit will not fire up, I disconnect all the external cables, manually carry the beast 1/4 mile to my office, blow it out, plug it up on the workbench and it works the first time. Not sure why but it has happened many times. Either being totally disconnected for 30 minutes, or the jostling it takes as I lumber to my office, or a combination of that and reseating all cables and RAM.
I found that the disk usually does not spin up when this happens, and after my little excursion, and some threatening remarks to it, it comes to life.
Leave your cover off and reconnect all cables. Turn the unit on and you should hear the beast fire up, the disk chatter a little, have no error signaling beeps, and see a green light somewhere on the motherboard.
If the unit will still not fire up, and if you cannot hear the disk spin up, remove the disk from the cage for awhile, then re-insert it, try again.
If you get past the BIOS boot, yet cannot get Vista to reboot, then try booting up in Safe mode. After the bios boots, hold down the F8 key until you get the prompt screen to boot in safe mode. If you can get to here, then continue in safe mode, then do a system restore back to a date when things were fine.
If you cannot get booted in Safe Mode, then load the System Restore disk, power off, then ON and do a system repair option. This just fixes/replaces any mangled software. If that doesn't work, then the last option is a full system restore.