I can see it under 'Disk drives' as 'Sony Eri Memory Stick USB Device'. (btw, i have no memory card inserted into the phone) and the drivers it is using are:
C:\WINDOWS\system32\DRIVERS\disk.sys
C:\WINDOWS\system32\drivers\PartMgr.sys
Hope that helps.
Edit:
I'm keeping the usb signed drivers in a folder on desktop. Just letting you know, maybe it will turn out to be a mistake.