WD Passport External Drive

Hi,use external drive to backup successfully
my HP Laptop with Passport. I have a 2nd computer used
for outside office. I'm trying to backup this computer to passport drive,
but when it backs up, it does to
same catalog as my home computer file folder...actually overwriting
the 1st home computer backup. I have been trying unsucessfully
to create a separate backup. I have 2 file folders,but cannot direct
to appropriate file folder, which have different names. I have VISTA on both computers.

Can someone assist me? Thanks

Maltesem