I am using Windows 7 and about 3 weeks ago I upgraded to Lightroom 3.
When running L3 for the first time, it upgraded the catalog from Lightroom 2 which had about 25,000 photos listed in it. It took ages but all seemed to go well.
Lightroom # started crashing when accessing files in the catalog and when developing images it crashed the computer a few times to the blue screen of daeth! First time I have that in a few years with Vista or W7.
My backup software started to have trouble also when it was running and explorer.exe would crash and the harddrive would heat up a lot. Computer crashed...
So try to run some diagnostic tools and the folder with the Lightroom 3 catalog in it caused even that to go heywire. I am thinking... what the heck!
After quite a few goes I finally got the properties on the folder and it was over 50 gig!!! I have not idea what was in there, there were 75,000 very tiny files and something massive in size, maybe 10 gig or more.
Finally I got them deleted (took a while and a crash or two) reinstalled Lightroom 3 and now it all runs smoothly.... so far.
And the moral of this story is... I don't know. But I love lightroom and hopefully if someone else has a problem this may help them out.