My Iphone 3GS Application Installation button is inactive and appears to look like that 'Install...' instead of full button whenever I click on it to install new application programmes like Free BP Tracker etc. It does not even give msg to say why. Why?

This is sometimes a symptom of trying to install an application on a phone that requires a more recent version of iOS than the phone is running. Check the iOS requirements of the app in the app store if you aren't running 5.1.1 to see if that might be the problem. If it is, you'll need to update your phone in order to install the app.

The 3GS can be updated to the latest iOS (5.1.1) and the 3G can be updated to 4.2.1. If you want to update your phone, first import all your photos and videos to your computer for safekeeping (see http://support.apple.com/kb/HT4083). Then back up your contacts. Then connect your phone to your computer and open iTunes. iTunes will probably automatically prompt you that an update is available. If it doesn't, click on the name of your phone on the left sidebar, then go to the Summary tab to the right and click on Check for Update and follow the prompts.

Also be sure to back up your phone when given the option. (I suggest you sync with iTunes before updating, which will create a backup).