Vista Premium & Mac OS X Login

Just got my Dell M1210 with Vista Home Premium today. Can see my iMac with
Mac OS x Tiger 10.4 and my HP PC with XP just fine. However, whenever I try
to connect to the iMac I get an error "Login Unsuccessful. Windows is unable
to log you on. Be sure that your user name and password are correct". The ID
and Password are correct! The HP and the Dell can access each other just
fine. And, the iMac can access the HP with no problems. Have had the iMac
since last October and never had a problem accessing the HP PC or an
additional HP Laptop I have. Anyone have any idea what might be going on
with Vista here? BTW, I have both disabled and reenabled the Windows
firewall and tried logging in both ways, with no luck either way. I need to
print to a printer connected to the iMac and get some data files off the
iMac. Any help would be VERY MUCH appreciated. No, I have not attempted to
contact Apple yet.