All I can think of is you could download MSN Messenger 7.5 again and Messenger Plus! and select to password protect them. Then download WLM and Msg Plus! Live. Which would be annoying.
What I would do, is find a program which password protects folders/files, like
AutoKrypt 7.03 or
Password Protect Folders 1.0 and use that to password protect your
old chat logs.