Just thoughts, can't guarantee anything:
The first thought is about the browser. Which one are you using? I've seen IE having issues with some sites. I suppose it's because each of the versions of IE acts differently compared to the standardized net site code.
Try resetting the browser you're using. Also reset IE, which is the same as Internet Options in Control Panel. They interact anyway on some level.
You say your computer is "fully updated". How fully? I'm not sure about whether the Optional Updates could have anything to do with this issue or not, but were talking about Windows who is a woman's mind in a female body. Computers are feminine, if you haven't already noticed...
I've noticed that some sites don't even ask for saving the login information and some of those which do, don't automatically fill it. On my computer it might be because I don't allow autocompleting forms and my cookies get wiped off every time I exit the browser. This site works with those settings, though.
Sorry, I can't give you any better advice than this.
Oh, yet one thought: If you feel the login has been working automatically at some date, you might have a Windows Restore point allowing you to test for potential update causes.
Addendum: A quick search revealed you're not alone with that problem. It also seems to affect all major browsers. And there were no real answers in any of the threads I read.
A couple of last tricks: First export your passwords if there's many of them. If only a few, skip. Install another browser, don't import any settings. Test if it works. If it does, use it. If not, you've at least tried. You can also try to create a new profile in your browser. In IE resetting does the same.
