Select the device you want the computer to boot from. To boot into Windows, you select the "Hard Drive".
However, if that has just started happening, the hard drive may have failed. You can test it with "Hitachi Drive Fitness Test for DOS" which loads from a bootable CD. You make the CD from the "CD Image" you'll find here: http://www.hgst.com/support/downloads/legacy-downloads#DFT