Based on what you wrote the Windows XP host probably has a default route for all 0.0.0.0 networks via its 192.168.10.1 gateway.
You can inspect this by doing a:
netstat -nr from a command prompt.
In order for the responses to egress the proper interface on the XP client you will need the 0.0.0.0 route to point to the 192.168.20.1 gateway.
You can delete and readd the 10 network interface on the client so the 20 network becomes the default or manually delete the default route and add a persistent static route via the 20 interface. Remember to use the -p switch on the route command if you want it to remain persistent across reboots.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...