Issues regarding website access

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...

Boot an Ubuntu/Knoppix CD and see if that can access internet or not - if it can, it might be a Windows issue :D What random "security" tools are installed, one of them might be blocking access :p

What error messages does she actually get though? If both your mum's and your PCs are Windows, you could try "ipconfig /all" in a cmd window and compare settings for DNS servers and Default Gateway :p If it's OSX, I'll assume the linux shell command "ip addr" works, but you have to decode the information a bit yourself :p Try pinging the default gateway, if that works, try pinging various websites and see what happens :p

yp is just a different server in that domain - www is not mandatory at all for websites :p

My computer runs Windows Vista, and hers runs Windows XP. I use AVG, she uses Norton.

But yeah, I'll check out the settings.

And where would I happen to find an Ubuntu/Knoppix CD? :p

www.ubuntu.com and www.knoppix.com :monster: