You can have up to a combined total of 6 devices, computers or sony readers authorised to 1 account.
For me I have 3 computers, my own computer twice because I forgot to deauthorise it before I reinstalled Windows(must remember to do that next time
) and my parent's computer plus my Sony reader. Both computers log in fine to my Sony account.
I reckon it might be a Windows firewall or Anti virus problem. Sometime it does take a couple of minutes to log in if the bookstore is running slowly.