Putting wider wheels on a hybrid bike I've recently had a bike donated to me by a friend so I can start commuting to work. It's a hybrid style bike, and uses 700x32C tyres. My ride to work is along a pretty bumpy, muddy canal path and my feeling is that the tyres currently on the bike are a bit too narrow for the terrain. I'm very new to cycling, and particular bike maintenance, so this might be a dumb question, but can I replace the wheels on the bike with something wider? If so, what measurements should I take from the bike to make sure I get something that fits?
Frame clearance can be a problem, especially if the mud is sticky and the tire too close - it can clog up quite badly if you go too wide. If you have V brakes, these can also, however my experience has been worst case is new brake pads. 

29" Mountain bike tires are 700C. (As Always - http://sheldonbrown.com/tire-sizing.html has the answer), so you now have a wide range of tires to choose from. The web page also has a great chart for tire width vs Rim Width compatibility. With a bit of luck you will not need new rims, just tires. 

If in doubt, take the bike to the guys at the LBS and have a chat with them. They will put you right on local conditions and will hopefully know the route you are riding and give good suggestions.