Grasping at straws, on the off-chance that your video driver is at fault here, on the Hirens Boot Disk, you can go to
Programs -> Editors/Viewers -> Notepad++
to start up a Text editor. Once open, go to File -> Open and open the file "Boot.ini" that is located in the root directory of your boot drive. Find the line under " [Operating Systems] and add the switches "/basevideo /sos" (without quotes) to the end of the line. Save the file (File -> Save) and try rebooting your machine. Those switches should force your machine to boot to standard 640x480 VGA video independent of any driver that you have loaded. If your machine comes up, you should reinstall the video driver for whatever video card that you have (visit your computer manufacturer's website). If you do this, then you may need to re-edit the file back to the way it was before it will boot normally.
More information on the Boot.ini file is here:
"Available switch options for the Windows XP and the Windows Server 2003 Boot.ini files"
< http://support.microsoft.com/kb/833721 >
Hopefully, if nothing else, the /SOS switch will display which driver was last to load before the crash.
My only other suggestion is to use Hirens to back up your data before reinstalling Windows.
HTH,
JW