|If you have the second partition in place just insert the WinXP CD while in the existing WinXP version. Choose custom install and choose the path you want to install to along with the language and the options desired. Just watch that you don't install to the same partition as your other version is on. |
That method will automatically create the dual boot situation.
You can choose which version will be the default, which doesn't require any user intervention to boot into.