First, check that your printer is set to A4 and not something else:
In your 'Printers' folder, right-click on the icon for your printer and select 'Properties'.
Exactly what you'll see at this stage varies from one printer to another, but there should be a tab labelled something like 'Page setup'. Click on it and check that the appropriate settings have been chosen. (If you need to change anything, do so, then click 'Apply' followed by 'OK').
If you're only losing the right-hand-side, of what you want to print, when printing from the internet (rather than, say, from Word), the problem lies with your browser. It's a well-known fault with all versions of Internet Explorer up to IE6. So, if you're using IE6 or earlier, upgrade to IE7 (which, theoretically at least, doesn't exhibit this problem) or, far better, switch to Firefox:
http://www.mozilla.com/en-US/
Chris