Hi, Warren, is it?
It sounds to me as though you are having problems signing into what has been nicknamed 'the synther'. This is the Photosynth program that installs on your computer that you use to create synths. Please understand that signing into the website here is separate than signing into the synther.
On the plus side we know:
A) You have an existing account, and
B) You know your password (otherwise you couldn't have started this topic
So we just need to figure out why the synther isn't able to talk to the website when your browser clearly can. Perhaps your firewall program has blocked Photosynth from getting web access?
Could you post the exact error message that you get when trying to sign in to the synther? Thanks! This should help the Photosynth guys narrow down the problem for you.
Also here are some possibly related topics: http://getsatisfaction.com/livelabs/searches?query=authentication&style=topics/