my ipod and iphone are not getting synced to itunes. help!!

my iphone and ipod are getting recognised when i unistall itunes but as soon as i install itunes ..for ipod it says an ipod can be detected but cannot be connected. And for iphone it shows nothing!! i have an iphone 4s and ipod nano 7th generation. i tried reinstalling itunes sevreal times and all the trouble shooting procedures! nothing helps!! what should i do?