How can I tell what the computer's overall memory usage is from Python, running on Windows XP?
You'll want to use the wmi module. Something like this:
import wmi
comp = wmi.WMI()
for i in comp.Win32_ComputerSystem():
print i.TotalPhysicalMemory, "bytes of physical memory"
for os in comp.Win32_OperatingSystem():
print os.FreePhysicalMemory, "bytes of available memory"