Obviously, you can buy any number of good graphics programs. But for free, you can get some de-pixellation with IrfanView, for example. http://www.irfanview.com/
Open the file, then resize it with Image / Resize. Change the Width (or Height) to four times the original value. Save the file and view it. The program interpolates to create three new pixels after each original pixel. The result is a smoother image. You should also try changing the original value for width to your screen resolution width. YRMV.
Programs which have a "smoothing" menu item, such as Adobe Photoshop or Photoshop LE, are available for purchase (on eBay, if you can get an unregistered copy). This automates the above process or accomplishes something similar.