Disable it from startup:
1) Start -> Run
2) type in msconfig
3) find the tab that says "startup" (is the last one)
4) uncheck the box for MS Instant messanger
5) as a precaution, also sign in to MSIM and in prefrences make sure it says, "do not start when windows starts", what that means is that MSIM won't automatically instal itself again in the startup option...
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DO NOT MESS WITH ANYTHING ELSE IN THE MSCONFIG SECTION, IF YOU DON'T KNOW WHAT AN OPTION DOES, LEAVE IT ALONE...AND AS A PRECAUTION, BEFORE YOU DO MSCONFIG CREATE A SYSTEM RESTORE DATE... I have not failed; I have merely found 100,000 different ways of not succeding...