For downloading images from your memory card to your computer, one can use the generic program from Window Explorer. However, it will not be able to view RAW files. Since Nikon has its own proprietory software for NEF, Nikon's version of RAW, it will produce the best result by downloading two Free Software from the Nikon website.
First, you will need to download NIKON TRANSFER, this will download your images from the memory card to the computer. There are two routes to take, one by connecting the USB cable from camera body to computer while another, by using a Card Reader that can be bought from either camera store or computer store. The advantage of the latter is that it will not use the camera battery to perform the downloading.
Second, you will need to download NIKON VIEWNX 2 for viewing the images. Nikon Transfer will automaticaly connect to the Nikon ViewNX 2 after processing your files. The latest version of ViewNX 2 allows for viewing Jpeg and NEF images as well as minor post-processing. It will automatically Auto-rotate your vertical images while Window Explorer requires manual adjustment.
http://www.nikon.com/about/news/2010/08 ... nx2_03.htmIf you wish to proceed into Post Processing of your images, you will need a software also which is not free, but many are available for 30 days free trial. The most inexpensive is Adobe Element. The most widely used is Adobe Lightroom 3. For those advance user, there is PHOTOSHOP CS5. Nikon has its own version of Nikon Capture NX 2.
I am no post processing expert but hope it is helpful. Good birding and photography.