So you're running Messenger Plus! 3, correct?
In that case, try the other hooking methods available in the Plus! Preferences > General Options > Miscellaneous > Messenger Attach Method: 'Alternate Mode' or 'Default Mode Forced'.
Also note that not only Smiley Central and similar Messenger addons might cause Messenger Plus! to fail to load, but also firewalls, certain internet security programs and even some anti-virus programs...
And note that if you have installed
Messenger Plus! Live (=version 4) at one point, then uninstall that first and reinstall
Messenger Plus! 3 since Messenger Plus! Live is not compatible with MSN Messenger 7.x and might disable/change the hooking method of Messenger Plus! 3.