[Post deleted - misunderstood original poster. Proper reply to follow shortly]
Right, now I've deleted the wrong instructions I told you a minute ago... it sounds like something has become corrupted with your fonts somewhere. Try the following: right click on the desktop --> go to Properties --> Appearance and click on "Advanced". In there, have a look at all the various items in the drop-down menu and see what font accompanies them. It might have been changed to something obscure. If it has, you can change it back by choosing another font from that menu. If this doesn't work, it sounds like it might be something a bit more serious with the fonts or Windows itself... let us know.