Cookies....remove all cookies and temporary files.
You can use CCleaner for that. Pay close attention while installing and UNcheck any offers of toolbars...especially Yahoo.
No need to use the Registry Cleaning tool and it has the ability to cause problems.
CCleaner - PC Optimization and Cleaning - Free Download
You may have software installed by the website. Check in add/ remove and browser(s) add-ons.
This time, if you are successful in opening a new account, obey the rules...
It may also be the case that entire block of IP addresses have been blocked by the site. Not necessarily
because of your account. It could also be that your computer has been blocked because of spam or other malicious content.
One place to check: Blocklist Removal Center - The Spamhaus Project