Cheapest of all professional sounding microphones suitable for recording grand piano are: Behringer ECM8000. They are so-called "omni-directional measurement microphones", and are fantastic at capturing the pianos full range. You'll need two to make it stereo.
They will need a power-supply and amplification; this devise is called "microphone pre-amplifier". The cheapest model will do, for example "M-audio Audio Buddy". You'll need one with two mic inputs, which the one above has.
You'll need two XLR microphone cables.
You'll need a "line-in" on your computer. If you don't have that, you'll need to buy a sound-card. For example "M-audio Transit" It's a USB device.
You'll need a recording software, for example Goldwave, which is a share-ware program, so download it for free.
That's it! Very cheap stuff, and very easy to use! You can only buy them in real music-stores, though.