Cannot map network drives

I am trying to map our network drives on a PC that
was just returned to us formatted with Windows 98.

I can browse the network, but when I try to connect
to the Windows NT4 server it asks for a password and
tells me that the password is invalid, no matter what
NT password I try to use.

We have a Workgroup, no domain.

I can connect to the Internet and as I said can browse
the network.

Can you tell me what might be wrong?

Thank you,

Sandy M