emails not downloading

I have been having this problem for the last couple of days.
My ISP server is "TADAust" which is for people with disabilities.
Up until now I have received all my emails without problems.
2 days ago I went on to check my mail and this notice came up on screen:
"Warning! Unable to write the email to the mailbox. Make sure the file system allows you to write priviledges and you have enough disk space to copy the mailbox. "

One email is shown as having been downloaded and when I delete this it seems to regenerate itself and the cycle starts all over again.
I am running Windows '98 and using Mozilla Thunderbird to open emails.
I have AVG on which usually stops any viruses (have ran a check and have found nothing) getting in and I have also ran a check for spyware so I am stumped.
Can anyone help with this problem.

 

http://home.att.net/~cherokee67/unablewriterror.html
The above link is from Ramona's solution. It should be applicable to TB.