I am using a USB scrolling mouse (XLR8 from Interex) with W98R2 on a new Athlon 700 PC (I think the MB is Microstar, but am not certain). The PC originally came with a PS/2 mouse, which I used while installing everything, then installed the USB mouse and removed the PS/2 one.
W98 now occasionally 'loses' my USB mouse drivers. i.e. I boot, and the W98 user login is displayed, and the mouse does not work. I then go into Add New Hardware and it says there is an unknown device under the USB controller. If I re-install the driver and reboot, it's fine.
I can't seem to see a pattern in this, except that it sometimes happens when I start typing in my windows password while the pointer is still an hourglass (but also can still happen when I sit still for minute and let it settle down, so this may be a red herring)
I should add that I was supplied with a Mac version of the mouse drivers by mistake, then my supplier downloaded the Win98 drivers from the internet and emailled them to me.