Sounds like you either have a browser configuration issue, or the login script for this site has an issue with the version of the browser on your Vista machine.
It's not a Vista vs. XP issue, it's either a browser issue and/or an issue with the login script on the site.
If you are the admin of the site in question, then you're going to have to check your code and Microsoft's documentation, since I'm assuming you're using Internet Explorer. So setting aside the fact that an additional security question is completely useless when using a browser as insecure as IE, where malware can easily read whatever you type into any text field, you'll need to figure out if there was some API change between versions of IE that is affecting your logging in. And if you're not the admin, then you'll need to contact whomever IS in charge of that site and hope they care enough to fix it.
When I type in my logon info at a website, I am supposed to be prompted to answer a security question BEFORE I can log in. Win Vista Home Premium skips that question and logs on to the website. My Win XP SP3 system DOES prompt me for the security question before it allows be access to that website.
My Vista Home Premium computer Internet setting is to NOT save login information for forms. I am the sole Administrator of this machine with Administrative Rights.
How do I make the Vista machine ask me for the security question as does the XP machine BEFORE it logs into a website?

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