The reason you feel it is slower than XP is because it is.
Almost nobody needs 4GB of RAM and XP will use 3.5GB of it NP.
With 2GB of RAM and XP I can run 4 security programs (one is blocking 1,169,864,545 IP #'s),6-10 torrent uploads/seeds and play Frontline:FoW without lag and while only using 49% of my RAM.
I never agreed with the idea that a 64 bit OS running nothing but 32 bit programs had anything to offer.
DX10 is runnable on XP...look here on the forums or use Google.