How can I enumerate/list all installed applications in Windows XP?

Check out the Win32_Product WMI (Windows Management Instrumentation) class. Here's a tutorial on using WMI in Python.

If you mean the list of installed applications that is shown in Add\Remove Programs in the control panel, you can find it in the registry key:

HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Uninstall

more info about how the registry tree is structured can be found here.

You need to use the winreg API in python to read the values from the registry.