This appears to be a common problem. I also cannot sign in on messenger 7.5
but have no problems at all on messenger 7. I am told I either have no
internet connection (wrong! I am connected!) or it is my firewall blocking it
(error code 81000306). I use Norton Internet Security and the firewall
setting for MSN messenger says Permit All! Lengthy instructions from
microsoft help havent solved this problem yet, and I also find that after I
have downloaded 7.5, my pc slows down drastically and some programs stop
responding altogether! I have none of these problems on 7.0. I hope someone
can help me sort this out! Thanks.

PS I have also noticed that I cannot sign in on MSN explorer. Internet
explorer works fine, but the sign in screen on msn explorer just stays
permanently on the flying butterfly!