Error 203 is a limit on how many trial accounts you can create from the same IP - if you have a static IP from your ISP, you cannot "reset" it (hence static) - if it's DHCP dynamic, you can pull the power to your router, wait an hour or so and plug it back in, you will most likely have a new IP.

Your best bet though, is visit a friend you trust (ie who has antivirus installed, and updates his OS), borrow his PC, log into the wow website and create your final trial account. Then, when you come home, log on again, and change your password (just in case).

When I made mine (I have 2 sets of 5, 1 set is retired, damn you RAF for appearing 2 weeks after first set was created), I both times hit the limit on the 4th or 5th account, so I had to dial my VPN into work, and use the server there to create the last accounts. Worked like a charm.

If none of these options are available to you, you have to call Blizzard and explain you're trying to create 6 accounts for multiboxing, but you receive an error 203 on the 6th account. They can unlock your IP. Or just wait a day or two, it resets itself.