There is no version of DX10 for XP.
There is a hack, though, where a couple guys tried to create a bridge that mapped DX10 API calls into DX9. There was an awful lot of talk about it at one time. Unfortunately, that's no longer being developed or supported, and has been "donated' to the community". Depending on where you stand on the issue, that either translates to
"someone else will make it work and screw MSFT", or perhaps
"failed and bailed". Personally, I don't care either way... But rather as a matter of security don't knowingly download hacked anything off of the web.
Regarding removing it - I thought there were instructions on the developer's site to do that. In general, though DX isnt' something that normally gets installed and uninstalled. I don't know if you would have to do a complete reinstall. Perhaps a Repair Installation from your XP CD.
You can also try re-installing the runtime directly:
http://www.microsoft.com/downloads/details.aspx?familyid=2da43d38-db71-4c1b-bc6a-9b6652cd92a3&displaylang=en