Windows XP SP3 release candidate updates usbport.sys from SP2, meaning all of the existing patches and replacement files for changing your USB mouse polling to 500Hz (from the default 125Hz) no longer work. Using them will either fail (complaining of the wrong file) or will cause your USB ports to cease to work in XP.
Luckily, the line of hex code in usbport.sys that relates to the polling rate is only slightly changed from SP2, and can easily be tweaked with a hex editor.
Default hex string:
XP SP2: 3C 08 73 09 C6 86 0A 01
XP SP3 RC: 3C 08 73 09 C6 86 0E 01
One hex character difference. Luckily, we leave this character alone when patching.
500Hz hex string:
XP SP2: B0 02 73 09 52 8C 0A 01
XP SP3 RC: B0 02 73 09 52 8C 0E 01
To do this, you need to use a hex editor (xvi32) and edit the file in Safe Mode. Not only do you edit usbport.sys in C:\WINDOWS\system32\drivers, but you must also copy the patched file into C:\WINDOWS\system32\dllcache (hidden by default, edit your folder options to reveal protected OS files).
Once you've got the patched usbport.sys in both the drivers/ and dllcache/ directories, reboot and you're good to go.
To check your mouse polling rate, use the dx_mouse_timer_dialog utility.