problems syncing apps with iphone

my device is iphone 4(ios 6.1.3). for the last 3/4 weeks, i have been facing problems while syncing apps with my device. after connecting iphone to my pc, when i press 'install' option of the apps and apply it, they don't get installed on my iphone. but they show 'remove' option opposite to the name of the app. that's the confusing matter. because 'remove' option is only shown when any app is installed. i didn't get any result after unplugging phone, quitting itunes, then again plugging phone. i did a hard reset on my phone. but again no result. plz give me a suggestion what to do. it occurred even i was in ios 6.1.2. i use itunes 11 on windows 8.