For loading Windows, Something like the WD velociraptors. More expensive than most desktop HDDs but cheaper than SSD.
For just storing data, something like the WD Caviar Black range, Samsung Spinpoint F3 or Seagate Barracuda 7200.12 are all quite good and reasonably priced.
This is an excellent answer, concise and specific. I'm looking into the WD Velociraptors myself as a boot drive, and supplement it with a tiny SSD for Intel Smart Response Technology, which is essentially a fast disk-caching facility.