can't login at certain sites

For two days now I have been unable to sign in at certain websites. Two of them are my email and my bank, so this is fairly urgent. I wait a long time going nowhere and then get a message "The connection to the server was reset while the page was loading."
At one of them I saw a one-time message that I needed Java version 1.4.2 or higher. I have no idea how to check what kind of Java I've got (being a bit of a noob), or whether this relates to the problem I'm having.
I tried all the usual remedies and in the end did a clean reinstall and re-format (something I was planning to do anyway for other reasons). The organizations in question tell me there is nothing wrong with my accounts. My provider tells me there is nothing wrong with my connection.
Running Ubuntu 12.04, but I have the same problem on my netbook running 10.04. I tried it on somebody else's computer (on my modem) running Windows 7 and signed in just fine. Then we tried it on an old machine running Win.XP and I couldn't get in.
The problem started suddenly on Tuesday eve. on these sites. A couple hours earlier I'd been signed in successfully on two of them.
PS I just tried to post this from my computer and couldn't do it, though I could sign in at Ubuntu Forums OK. Same problem. I'm sending it from another machine.