Hold down "Alt/Option" while booting (Right after the start up tone). You can select Windows or OSX here.
Also, on both the Mac side under System Preferences, or on the Windows XP side under Control Panels, is a Startup Disk preference/selection.
You can use these to set which system you boot into.
For me, since I usually use Mac OS X, I have both set to Mac OS X, then use the OPTION key to select Windows XP during boot.
To the OP, AFAIK you cannot have multiple Windows OSes such as XP and Vista on the Bootcamp partition. You can have either, but not both installed.