When my website is displayed through explorer on a 17inch monitor compared to a 15inch, the pages align to the left instead of centrally & also some items on the home page are extended or in the wrong place. What is going wrong??

Answer: 

it is not the fault of a 17 inch screen. Typically computers with a 15 inch screen have their video card set at 800 X 600 Pixels so that everything is viewed in that many pixels. It only looks centered because the site is using the entire screen.

Computers with a 17 inch screen have newer video cards with resolutions of 1280 X 1024 pixels  . This then allows your site formatted in the smaller resolution to spread out and reveal things you would have missed on a smaller screen with less resolution.

On a PC computer you can "Right Click" the desktop background and then a menu pops up. Click on the "Properties" menu link and in that you will find a lot of choices but one of them is "Settings"/screen resolution and will probably be set at 800 x 600. Try it and see if it will go to 1024 X 768 - this will make the print much smaller on your 15 inch screen but everything will work fine and you'll now be able to see what bigger screens see.

If that isn't enough to help you figure it out find some local computer GURU and have them help you with this specific issue of adjusting your computer screen.