Installing OverPowerOnline on Windows XP (and possibly Windows 2000 as well) is very simple.
- Download OverPowerOnline from here
- Double-click "OverPowerOnline_v2.5.3.exe" to extract it, it doesn't matter where you extract it to.
- Open the folder where you extracted it to, there should be a OverPowerOnline_v2.5.3 folder.
- Double-click "System32 Package.exe" to extract the needed DLL files to run it.
- Double-click "OpDeck.exe" to load the deck-builder application.
- Double-click "OpOnline.exe" to load the game client. Make sure to go to Tools > Settings and set the User Name/ID to something other than Leon.