Actually i have noticed a big difference between win98 and XP - I have both installed on my rig and while 98 works smooth XP stutters like hell SP and MP.
My brothers P4 1.7G with XP does the same.
I have AthlonXP
1.4Ghz and GF4 4200Ti 128M and 256DDR
My brother has P4 1.7Ghz and GF2 Mx 64M and 256DDR
it looks like thrashing or swaping or whatever u call it when it doesn't stop reading. and it alsodoes that an low res low detail...
PS - both versions do that