Well, if you really want an answer to the problem, I'd suggest reading the thread on Usenet from the link I posted in my previous post. Apparently some people are having cable-breakage (sometimes inside the mouse housing, so it can't be seen) on these mice.
I'm not sure why that would affect only the USB signal path vs. the PS/2 signal path, but they called MS and went through some troubleshooting, and then received new versions of the mice and apparently the problem disappeared.
So, my point is that sometimes it pays to look a little further than the obvious when trying to troubleshoot a problem such as this. In other words, it could be a bad mouse that just happens to work ok on the PS/2 port. I have a friend who's using one of the MS optical mice with Windows 2000 and he, too, is having intermittent freezing on the mouse pointer. So far, all the problems I've read about have been with Abit boards of one type or another, too, but again, that's just anecdotal.
I don't mean to tell you what to do with your mouse, and I hope that's not the way it's coming off. The bottom line is you want a mouse that works, I know. But if you bought a USB mouse to use specifically because of the advantages of USB, then it bears further investigation. It's under warranty. You're entitled to support for it. It should work the way it's intended. Good luck...