What version of Windows are you running Gav?  XP, Vista or Win7?  Also what version of browser are you running?

Can you just confirm its only in your web browser which everything is screwed up?

If its IE, go into TOOLS and make sure COMPATIBILITY VIEW does not have a check next to it.

Next, go into TOOLS and INTERNET OPTIONS.

Select FONTS.

Make sure the drop down box at the top has LATIN BASED selected (should be shown)

Make sure the window WEBPAGE FONT: has Time New Roman and PLAIN TEXT FONT: Courier New

If all the above is correct and same, then as a last ditched effort, go to TOOLS and INTERNET OPTIONS.  Select ADVANCED tab.

Down the bottom is a button RESET.  This will do a 'factory' reset on your IE options.  Worth a try.

Make sure for virus software is up to date and to a full scan.

If its happening all over windows (ie not just internet browsing) we'll go to plan B.