Thanks for the quick reply guys.
NiLuJe - only change I've made recently has been to add USBNetworking. I haven't used it yet, just installed it on the Kindle. I was hoping an uninstall us USBNetworking would 'fix' it - I'm guessing that's not the case?
I just did a search on 'corrupt userstore' and this thread is the only thing that came up. Any tips on what to do? My Kindle isn't restarting at all now (stuck on the USB Drive Mode screen).
Phew - got it connected to my computer, so I'm copying all the files in my 'documents' folder over (I think the computer they were on is gone now)... Now what do I do about this corrupt userstore?