Let me elaborate on your situation, just so we are clear:
1) You used to have Windows ME installed on your current computer system.
2) You installed (upgraded or clean installation?) Windows XP onto this
system.
3) You are saying that "somehow" Windows XP got uninstalled?
You mean your hard drive was formatted?
or
You mean you did this?: http://support.microsoft.com/?kbid=303661
And now you cannot locate the CD that you originally installed Windows XP
from onto this computer?
Well, you could be in trouble. If you bought that CD at a retail outlet (it
is not an OEM copy - meaning it came in cellophane backed by a piece of
cardboard.) - then you MAY be able to do something:
If XP has been uninstalled, then you'll have to use an XP CD of some kind to
re-install it. How did XP come to be uninstalled - I'm not sure that I
understand you?