I agree it sounds like a problem with the install file – I’d suggest download it direct from http://us.dl1.yimg.com/download.yahoo.com/dl/msgr75/us/ymsgr75us.exe – preferably using no download accelerator or manager, AND even with your antivirus program disabled while you do the download.
Once downloaded, run this install file, and cross your fingers.
TY Eagle_Kiwi – I had EXACTLY this problem (also had downloaded from yahoo so was using the msgr75us.exe which didn’t work). Used the link above and downloaded (no accel, firefox ‘manager’, and did not turn off Norton AV). Installed in less than 2 mins. msgr75us.exe was only 320K so should have smelled a rat but … Thanks again