Hmm. InPrivate IE more or less removes your history, cache, cookies, etc. You can try clearing Cookies too (but that'll clear all your saved passwords, so make sure you know them before you do that XD). If that doesn't work either, then I really don't know. You could try clearing the other options too, but I don't think they're causing the problem.
Have you tried using another browser to access the forums? One that you've never used at school on your laptop? If your cache was preventing you from logging in, you'd be able to if you used another one.