so does your mouse freeze but the rest of your system still works i.e. you can use keyboard shortcuts and stuff? if this happens only during browsing the web or downloading things it is probably a problem with your LAN card not having the right drivers. this happened to me as well on my old system

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iMac ::: Mac OS X 10.6.1 ::: Intel Core2Duo 2.93GHz ::: NVIDIA Geforce GT 120 256MB ::: 4GB DDR3 RAM

Milanca Project ::: Mac OS X 10.6.1 Retail ::: Vanilla Kernel ::: Intel Core2Quad Q6600 2.4GHz ::: NVIDIA GeForce 8600 GT 256MB ::: 4GB DDR2 RAM ::: GIGABYTE GA-EP35C-DS3R