Directx 9c is just the last version of directx 9 which was produced, the "c" has nothing to do with a test version.
And I would suggest you do not try directx 10 on XP unless you have made a system backup, I do not know what sort of risks there is but some say it may cause problems.
You can find a link if you just search for "directx 10 for xp"

Oh and I thought I would just point out DirectX 10 is not the latest version if you where wondering, there is now DirectX 11 which is awesome. Unfortunately only for Windows Vista and Windows 7 :redface: