I run trackIR4 pro with amd athlon64 3200 and gforce 7900GS, before 6600 (no GT). I did not notice any frame rate hit, so i guess if there was a hit it was minor.
Are you sure that you didnt change something else too? Be it resolution, file scanning av software or whatever?
I have the trackir software window sitting on desktop, but not the virtual head and settings screen. Do you have the virtual head & settings screen open on desktop, maybe causing trackir to send info there AND to AH?
Thinking a bit more, i have a few ideas that might help...
Well, maybe it does bad on AthlonXP, anyone knows?
Also maybe you run out of RAM with trackir on where you had just enough before?
Maybe it runs bad with old graphics drivers like the one which is best for your card?
sorry i cant be of more help, but i can assure you its NOT an trackIR general problem but has something to do with your setup.
ciao schutt