Issues regarding website access

[Momiji]

Okay, my mom's computer can't access PayPal, eBay, or Amazon, but mine can. This has been going on for almost a year now.

The riddle of this is trying to figure out what is denying her access. She has no viruses or spyware or anything (she makes sure of it by overprotecting her computer), and even when she turns the firewall off, she can't access any of them. How do we remedy this? We have Verizon FiOs, and she uses MSN to use the internet (I use Firefox). I did install Firefox on her computer in hopes of getting past the issue that way, but it didn't work.

I ask you, my techy friends, what do I do?

EDIT: We figured out backdoors into Amazon and eBay. Paypal is still a mystery, though...
EDIT²: What does 'yp' stand for? She found a way onto Amazon through yp.amazon.com, and it worked. However, I'm worried that this isn't secure...