You can visit their forum through a proxy server,
I think there needs to be a reminder of the original question.
The basis being. The OP would like to access a forum under a alias to be able to reply to posts and not be recognised as a already registered member by the admin staff.
With using the same computer this will not be possible because the admin staff will be able to see that the IP address also corresponds with another registered member.
Using programs that hide your IP address are only of use for general surfing so as not to leave a trail of your personal IP address.
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