OK the first phase - PXE/TFTP is done.
The problem you are having is with the ISO itself.
Have you used "PowerISO" or something else?
My guess is that you have lost the Boot information stored within the ISO file.
Try making the Custom Hiren's Mini XP ISO again.
- Download Hiren's Boot CD 15.1
- Extract it to a temporary folder (C:\Temp).
- Delete HBCD\Dos folder in order to save space.
- Delete HBCD\Boot folder in order to save space.
- Extract HBCD\XP\XP.wim (using 7zip) to C:\Temp
- Delete files XP.wim, X, XP.BIN in HBCD\XP folder in order to save space.
- Open Hiren's Boot CD ISO file (using PowerISO) and delete all files from it. (Do not close PowerISO with Hiren's Image!)
- Add all files from C:\Temp to PowerISO Console and save as hirensminixp.iso (we do that in order to save the boot information from the original ISO)
- Move hirensminixp.iso to /tftpboot/er/plugins/hirensminixp/