Perhaps it is software based then. I must also note, I've noticed this interference without the mouse plugged in. The machine is usually my HTPC, but I'm using it as a main PC for the time being, hence having keyboard and mouse plugged in. Here are the specs:
AMD Athlon 64 X2 4200+ @ stock under an Alpenfohn Panorama (kept nice and cool)
GA-MA78GM-S2H v1.1 on BIOS F11 (latest)
2x2GB OCZ Gold PC2-6400 @ 1.8V, 800MHz (stock)
Antec Micro Fusion Remote case, with Antec 380W PSU
Samsung F1 1TB
ATI Radeon 4350 256MB (Asus, passive)
Hauppage WinTV Nova-T 500 Dual DVB-T tuner (PCI)
LG BD-ROM/HD-DVD combo drive
Logitech G11 Keyboard
Razer Deathadder
Samsung SM226BW, connected from HDMI (card) to DVI (monitor).
Windows 7 Home Premium x64
All software is up to date, including drivers. Audio drivers are latest from the gigabyte website. I have yet to play around with older versions, or drivers direct from Realtek for this. I recall the Realtek drivers not installing anyway. As you can see, no overclocking on this machine, just after reliability. Oh, lastly, there is a USB drive plugged in for readyboost.
So, I'm going to try audio drivers again, I might also change the USB polling rate for the mouse too, and see if any difference is observed.
Thanks for your efforts.