Shoutbox

if you have an error message, look up the error number on official pages like http://messenger-support.spaces.live.com/

(and not on sites which only say to reinstall or reregister some DLLs, which they essentially suggest for every error)

anyways, about error code 80072745:
http://messenger-support.spaces.live.com/blog/cns...8B853F!13931.entry

See if that helps, if not, report it to Microsoft and make sure you provide as much details as possible, including saying what steps you've already followed, etc:
https://support.live.com/eform.aspx?productKey=wlmessenger