An intermittent freeze is known in Vista for certain Intel chipsets if you do not download and install the Intel Chipset Software Installation Utility at Intel® Chipset Software Installation Utility - Intel® Chipset Software Installation Utility support

Confirm you have an Intel chipset before downloading.

It could also be caused by any number of reasons. Since a device conflict is unlikely, you may want to check your CPU and GPU temperatures. An overheating problem can cause your CPU to lower its clock speed rapidly, causing a problem. A defective network card could also cause this problem.

Also check your disk drive for any bad sectors which may be causing your system to lag on reads or writes.