Default Phone not recognized by Windows on two PCs

Hi,

Let's start with the beginning: I've messed a bit with the rooted phone: deleted some seemingly useless stuff, with as a result the inability to install apps from the Play Store and even to check the storage status, so I did a hard reset but it didn't really fix it. Seems like I need a new ROM, which I wanted all the time (a custom one as we're talking)
So I've allowed Android to unlock bootloader on my Xperia T with Jellybean 4.1.2 and apparantly it rooted fine again (these instructions worked fine) and now there is the SuperUser app installed since it was forced in some way.

Now I want to follow these instructions, but step 13 brings bad luck!
When I connect the phone to the pc with either the volume up or down key, Windows says the USB Device malfunctioned the last time it was used and might not be recognized, or it doesn't react at all, except for the connection sounds. Going to the next steps doesn't bring relief...
But I can acces the phone perfectly when it's on and connected, when it's off it shows "D:\" in the Device Manager in Windows.

So I tried to update the driver manually, but when I go there it says there is no compatible installation file (while still following the instructions above)

I use Windows 8.1 on both PCs.


Please help? :/