Sorry guys,
Have been away for 24 hours. Got my DPC latency down to 6 microsecs after taking tips from this forum and a pc gaming fiend/friend - did that by disabling shedloads (HPET etc) in the bios.
The usb mouse is a ****ty little £3 from Asda (Walmart).
I am using USB 2 ports and move the interface and mouse around to see if that helps - but it doesn't.
I haven't loaded any drivers or software for the mouse - just legacy win xp ones.
I can easily get a new ps/2 mouse off ebay and might try that for now.
Might treat myself to a tracker ball for Xmas :-(
The motherboard is a 2 yr old Gigabyte GA-M720-US3 (AMD Phenom) with latest bios update.
Running 2 big monitors with all accelerations turned off for best performance.
Thanks to everyone for their kind help.