I've already helped someone upgrade from wm5 to wm6.1
You got wrong version software, you need i600XXGD1, this is the first step to do. Afterwards, you will need to run the i600uXXupdater.exe to get official wm6. From there on you choose wm6.1 nokser rom and you're done, read below carefully and use windows xp
you need first to update your s/w version to i600XXGD1, here you go
http://rapidshare.com/files/33331938..._i600XXGD1.rar
or
http://www.mediafire.com/?b1govxm3t0t
I guess you already have the modem drivers installed. Well, don't forget to disable usb connections in active sync in order to perform a good flash.
Once you're done with this step you're ready to update the phone to wm6.
Change the pre-settings to dutch version this<way:
Enter in the "Admin setting" menu:
*#1546792*#
Enter in the "Preconfiguration" menu:
Operator Code *#81230*#
- select "Netherlands", the phone will reboot a couple of times
- run the "i600UXXupdater.exe" and follow the instructions.
When finished you'll need to enter admin settings again to choose your country
Then use the nice guide provided by callumography to upgrade from wm6 to wm6.1