okay, do this.
go to start>control panel>mouse
click on each tab and check the settings. See if anything there looks like it might be causing the erratic behavior. Then, on the last tab, 'hardware', click on the troubleshooting icon. I went through some of it, the usb option, but I don't know what kind of mouse you have. You may need to uninstall, then reinstall your mouse.
I check the MS KB for articles, but only came up with results for laptop computers.