Strong - Firstly, it is possible to do mac address blocking, though I rarely ever hear of many places that would actually do that... Anyways, it seems to me that it is possible that you are using Internet Explorer and simply need to update to IE 8, they could be using CSS2, in which case IE7 does not support...(though I do not know their errors so yea, they all could be right and your IP is blocked) Also if you have Vista, I sure hope you have the 32-bit system if you want Malwarebytes to be of any help to ya...
Also HiJackThis, though it is a great utility for Windows 2k and below, you can find out more about your system using your system, then you can using HiJackThis...
If you really want to make sure you are clean, the only other software I have found to be more thorough then Adaware (which I see you have) is Spyware Terminator...
One more thing, you do also have to realize that your not dealing with stupid ass coders here, it is more then possible they have both your ip and account on block, this way if you try using a different ip to access your account, it will still be blocked... Also if they are doing Mac blocking, then you really are screwed... Try having your friend log in under your account, see if they still get the same error, or something of the sort, maybe even try on a public computer?