What's your video card?
What DirectX 10 games did you try?
I have a 8600 GT and Vista 64 bit. When I play games like Bioshock and Crysis, they automatically load DirectX 10. I have to add "-dx9" to the command line to run them in DirectX 9 mode.
You could go here
and download the latest version of DirectX if it's not already done.