Well it could be several things.
First off Try the sites under Explorer. IF they work there, then it is your browser.
Next if the first did not work (likely won't but DO IT anyway) Do a full spyware scan. When it finishes, boot into safe mode.
Then do a FULL spyware scan, finish, reboot and repeat in regular mode again.
Repeat this process so long as you seem to be getting fewer and fewer found.
If you eventually get a clean scan try again.
Sometimes certain Trojan Loaders that bring spyware programs onto your system can temporarily disable your encryption.
If it still does not work then your cyptographic services in Windows XP are likely broken. Give me a few and I will track down the KB for you.
Randy
*EDIT*
Tell you what, do this....
http://support.microsoft.com/kb/813444/en-us[This message has been edited by 8Ball (edited 06-26-2006).]