You can use all of the free and commercial editors on XP:
PowerShell Integrated Scripting Environment (ISE) ships with PowerShell V2
PowerGUI (www.powergui.org) is a great free editor with support for code folding and intellisense, and PowerPacks to make it easy to work with PowerShell by point and click
Idera makes another great editor (PowerShell Plus - www.powershellplus.com) with a strong collection of scripts that ship with it.
DevFarm makes PowerWF and PowerSE (www.powerwf.com), and editor and workflow tool for working with PowerShell
PrimalForms is an editor and tool to make UI in PowerShell, built by Sapien (www.sapien.com)