could be due to XP optimzing the programmes for better performance.
i have XP with a dual booting O/S with win98 on the other, and i find that the same games when played in win 98 are'nt as fast as in XP.
i can crank up the details in XP without any noticable difference but in 98 mode its sluggish.
i'm having the same problems as you are with GP4, with my old Geforce 2 gts 32mb card on win XP i could crank it all the detail levels up and get it running at 34 fps during practice with no other cars ( it slowed down alot when in a race due to the other cars)
but with my new Geforce 4 card it crashes to desktop, so i can only run GP4 in win 98 mode.
However in win 98 with my Geforce 4 card i get the same results as i did with my GF2 in XP mode, so i have spent £210 for nothing in regard to GP4
Nascar 2002 & GPL both run in 1600 x 1200 with maximum detail and 4 X FSAA with out breaking a sweat.