ok it's likely a conflict between Yahoo! Messenger's skinning engine and WindowBlinds. If you can, configure Windows Blinds program so it won't try to skin Messenger and set Messenger's skin to "Classic", which basically turns off the Messenger skinning engine. Click "Change Skin..." in the Messenger menu.
thats really odd its just a conflict im preatty sure there isnt much you can do about it. all i can think of is to use trillian which has AIM, ICQ, MSN and YIM all in one i love it. good luck tho and let me know if you find a fix.