Try this thread from Adobe LR forum, I had the same problem (solved): http://www.adobeforums.com/cgi-bin/webx/.3bc33df8
I posted this in another thread but maybe it should have gone in here --
Windows XP --
When I plug in a USB key or turn on my external USB har drive Lightroom starts up. Nothing is imported and if I click a jpeg it opens in my default viewer as expected.
I have checked assocations - not the problem
I have turned of the autoplay for each of the USB devices
Still starts when I plug in.
Any ideas as to how to turn off this auto program start up?