windows security

i have insight/comcast cable internet, and every time i try to check my mail,
a window pops up asking me to verify my username and password. i type the
same thing on my laptop that i do on my desktop, but the laptop will deny me
access. cable provider says it's not their problem, bestbuy (i just bought
my laptop from them last friday) says it's not their problem either. can
someone tell me how to disable this particular security question, and has
anyone else had this problem?