The cpu is the brains of your computer. Unfortunately, there are so many different models it can be very difficult to decide which one to get. The sweet spot right now in chips is the Intel Core i7-3770K Quad-Core Processor. The newer 4770 is much more expensive but only 0-15% faster depending on what you are doing. (Don't get me started about how intel doesn't really roll out faster chips if there is no competition.) This chip will run windows or mac without issues. It has a built in video card which is fine for basic web/email/movies use.
Get yourself a new CPU!