I have a Microsoft Arc Touch mouse with its own wireless dongle, but my issue has happened to me on a couple of other wireless mice as well. Whenever I access the network (i.e. copy files across a local network, download files from the web, etc.) my mouse gets laggy/skippy.
The heavier the network access the worse it is. I've tried moving the dongle around in case it's interference, but that hasn't helped. Any ideas? I am using Windows 7 currently.