My dad is having trouble totally accepting evolution. He brings up complex chemical reactions such as the Kreb Cycle and how half a Kreb cycle would be useless. I've tried to explain that for things to evolve, each intermediate step must be useful. I've managed to convince him of the eye but microbiology still stymies my attempts to get him to completely accept evolution. Any suggestions?

Another good question.  Half a Krebs cycle would be useless.  However, it's only useless if you are trying to get the products of the Krebs cycle as we understand its function today.  It is entirely plausible that the products of the component parts of the Krebs cycle did something useful for the organism that they arose in (in the past), and later extra steps were added, until we have what we recognise today.  The other important thing to remember is that as long as doing half a Krebs cycle is not harmful to an organism, there is no reason why it cannot be going on in the background, until it becomes advantageous.  Many similar arguments have been used to say that the flagellum of bacteria is too complex to have evolved.  This has recently been shown to be false.  With many of the componet parts of the flagellum having other uses.
The eye is easily explained.  There are two sorts of complex eyes in animals today.  camera eyes and compound eyes.  Compound eyes are seen in the arthropods, while camera eyes are seen in two very distantly related groups.  The vertebrates and the molluscs.  Even the simple pigment spots seen in flat worms and larvae of many animals are doing what our eyes are doing.  Sensing light and converting it into an electrical signal.  Basic eye spots for sensing light are seen across the animal kingdom, and arose once.  We have added lenses, and rods and cones to see colour and shades of grey, but underneath all of this, the genes that are involved in building  eyes, are the same in us, wasps, octopuses and worms.  The Pax6 family of genes specifies what cells in an embryo are going to become eyes.  The sequence of the chemical that make the gene (the nucleotides) are almost the same in mice and fruit flies, but not quite.  You can always recognise the mouse gene from the fly gene.  However, you can inject the gene from a mouse into a fruitfly, and it will make an eye.  The really clever thing is, that gene is in an environment to make an eye, but the environment is that of a fruit fly, so even though you are using a mouse gene, you will only ever get the fly eye! 
There are some very good books on this subject, and the evolution of complexity.  Kirschner and Gehart's 'The Plausibility of Life' is a good example.