Well, I partially agree with Franses...
I guess it works fine on your system.
If you would play Civ3 on a P3 with half that memory,
and used WinXP you would probably have caused a lot of lag.
Therefore I recommend NOT using WinXP if you have less than
a 1GHz CPU and physical RAM of 256 MB.
Experience tells me that XP drains far more memory than any
other OS, causing problems to several applications,
and even the operating system itself.
Proof:
(Tested XP on a box of 128 MB RAM, and 700MHz. And that was a real
plague)