Spacebar is used to press buttons in Windows. Its similar to left-click of a mouse. When you open WMP, by default the focus is on Play/Pause button and when you press Spacebar you are clicking the button which has focus - the Play/Pause button. Its not a WMP specific shortcut.
In WMP the Pause/Play keyboard shortcut is Ctrl-P
. If you want to customize the WMP shortcuts (for some reason known to you) you can download WMPKeys from http://wmpkeys.sourceforge.net/
. The installation and usage instructions are given on that page. WMPKeys installs global hotkeys, so even when WMP is not in focus you can press a hotkey to make it do something.