Browsing around on Microchips website, I found this page:

http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1406&dDocName=en533048&redirects=icd2Help

on troubleshooting ICD2 issues.  One item that caught my eye relates to 
complaints on this list in the last few weeks about users having their 
ICD2 work for a while - an hour maybe - and then quit.

One reported cause is that some PC's turn off power to the USB port when 
it is idle for some time:

Quote:

2. After a few hours in use, the PC cannot recognize MPLAB ICD 2. If the 
USB cable is disconnected then reconnected, the PC says that it senses a 
new device but cannot find the driver.

Resolution: The PC may be configured to put the USB port to sleep when 
it goes idle.

Go to the PC's Device Manager, and check the Properties of the USB Root 
Hub that MPLAB ICD 2 is attached to. Click on the "Power Management" tab 
and make sure the checkbox to turn off the USB hub is NOT checked.

Make sure when connecting MPLAB ICD 2 to the USB port that the power 
supply is not connected. MPLAB ICD 2 may not enumerate properly if a 
power supply is connected first, causing the Device Manager to appear.
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