I recently got hold of a beaten up, but working Sony Ericsson k750i phone. I'm planning a couple of mods for it, new case and maybe an LED colour change. However the first thing I did was hack the phone so I could upgrade its firmware to that of the w800i, basicaly changing the phone into a one with Walkman capabilities. The two phones are identical in makeup but come with different firmwares installed. Anyway I found this method on a forum somewhere, apologies to whoever I copied and pasted it from. I cleared up the spelling and grammar and present it for you here.

What you need:

-The exploit (found here http://www.megaupload.com/?d=MP5ZL5TQ)
-SEUS the sony erricson firmware upgrader (download at www.sonyericsson.com/support)
-W800.exe (Found in the Exploit)
-The CDA you want to use ( http://www.se-world.info/cda-strings )
-flash-driver (same as when you use SEUS for first time, see tutorial )
- 'cygwin1.dll' , you will find this in the exploit files.

What to do:

-copy cygwin1.dll to your windows\system32 folder
-EXTRACT CONTENTS TO A FOLDER NAMED W800
-the phone has to be cid36

Now step by step:

-Turn phone OFF and remove battery, sim card and USB cable.
-Replace battery (Do Not Turn Phone ON)
-after unzipping Double-click "W800.exe"
-Connect USB once you see the "30 seconds" message.
-A pile of data should scroll down the screen.
-On the blank line below the last bit of text type cda102430/6*, then hit Enter. *Note: here has to be your CDA you need. Take a look at the CDA string list for the one that you want.
-Now Type Q in the blank line below the last piece of text, then hit Enter.
-It should now tell you to remove the USB, after this you'll recieve a connection error. This is normal. close the program.

Now follow the SEUS-tutorial, your phone should be detected as W800.
Complete the update via SEUS, that's all.

NOTE: you do this at your OWN RISK!!!!