VPN Connection Problem

I'm trying to connect a Win98 PC through Windows VPN Connection to a server
to collect e-mail. I take 2 steps first I open a connection to the internet
then I connect through a VPN to the mail server then just click send and
receive on my mail application.

I have been succesful in connect for about 2 months but now suddenly a
pop-up screen is appearing that asks me for a username/password/domain.
Firstly neither the mail server or my PC is conneted to any domain, the mail
server is connected to a workgroup but not a domain and the username and
password is correct - I've set them and nobody else can!

I've tried creating a new VPN connection but no luck, I've tried a new
username/password but still the same pop-up screen.

Anyone any ideas on firstly why this pop-up screen is only appearing now and
how I can resolve it

Thanks

John