Sony Laptop no cd.

I have just recently aquired a older sony laptop (pcg-r505ts).
I want to install winxp on it. There is no floppy and there is no cd.
I have and external but the bios does not allow booting from external devices.
It does not allow booting from a usb flash and I don't have any other peripherals to attach to it.

I have already installed the os using another laptop, but it wont boot because it says the ntldr is missing. I does boot fine from my thinkpad though.

I have tried the bartpe method and followed the instructions down to letter on creating an xp boot key, but apparently when the bios says boot from removable media it is not referring to the usb key that I made.

I do however have a mediastick slot built in.
Is it possible to boot to a dos prompt and install it that way.

Any ideas would be helpfull.