I'm a Trillian Pro user for IM on Windows. Today Yahoo changed their IM protocol, and there is a bug in Trillian Pro 2.0, and I think the older versions, that can cause Trillian to crash when started if you have it auto-connect. On my old WinME craptop at home I'd get a Kernel32 error and would have to close Trillian. If this is happening to you, you may need to edit the ini file directly - since you can't use the Trillian UI to change the auto-connect setting if it crashes on startup. You'll have a file yahoo.ini somewhere - for me in C:\Program Files\Trillian\users\default. Change "auto connect=1" to "auto connect=0".
Drove me a bit nuts until I remembered today was the day for Yahoo's change, then I figured it had something to do with that.