thunderbird troubles...

right, just wondering if anyone out there can shed some light on this little conundrum thats cropped up. I use Thunderbird for my emails - have been for the past couple of years without problem - unfortunately the other day when I opened it up I received the following message:
"mail server responded: incorrect password...or maybe you don't have an account here"
At the same time I tried to send a message and got this:
"error occured while sending message. Mail server responded <myemail@whereever.com> does not match authenticated user name."
I run a few accounts through thunderbird; one from my service provider, a yahoo one and one for my website provider. I have not had any problems until just a few days ago.
now, I've not done anything to account for this, so after much rummaging around I took a backup of my inbox (in the profiles folder found in a path similar to c:\docs&settings\localsettings\application data\thunderbird) and got rid of the accounts by deleting them from thunderbird entirely (corruptions?) then I did a reinstall and put the old back-ups in where they came from. oddly though I try to delete all trace to Tbird and on a fresh install it finds everything straight away...most odd.
now, thunderbird wants to download all 2000+ emails in my yahoo email (those on the server) and has ignored the backups returned to whence they came. I still can't send and I still can't access one of my email accounts...

anyone any ideas and what I could try? please don't say outlook express...