can't shop online

For some reason, anytime I go to an store's website (best buy, circuit city, ect.) I can not buy anything from the site. I can select the item I want, but when I click Checkout or Puchase, it says "this page can not be displayed."
The same happens when I try to log in to my e-mail account and online credit card account. What could be causing this?

 

Could be a firewall on your computer. Could be cookies turned off. Could even be something like javascript turned off.

 

in internet explorer go to tools -> internet options. click the security tab. then click default level. see if that helps.

 

Cookies and internet security are set to their default levels and it still doesn't work. I have two firewalls and both are active, but they have been for a while and this is new. If it is javascript, how do I fix it? Also, could it be a virus or spyware?

P.S. I'm running windows xp home.