Windows itself doesn't show this anywher as far as I know.
You will have to run a utility like CPU-Z on the system itself to determine this.
If you have an Intel AMT infrastructure you can probably query this through that, but those are pretty rare in the wild. (I have never seen one.)
It is possible to query this via WMIC, but I don't know this is available on XP. A quick attempt shows it appears to work on Win7.
wmic memorychip get Capacity /format:list
Will give a line for each chip with the capcity in bytes.