I was having this problem a couple of days ago, even reset my password and all that jazz, in the end the only way I could log in was attempting to register as a new user and then it prompting me to enter my password again.
I am using a pretty prehistoric IE7 though. (No fault of my own)
It seems we've had a few mentions of checkout problems over the holidays.. There were a few issues we knew about with outdated browsers, but don't worry, we have a new full time member of staff starting on the 4th, who will be in charge of ensuring you all have the easiest, and best online shopping experience anywhere on the net in 2011