LR3 crashes on opening and system hangs

Question

Hi- I just got LR3.
I kept my Lightroom catalogues on an external drive.
First time I opened LR3 I linked to catalogue and when it was about 75% through upgrading, system froze, got grey overlay on screen and message to restart computer - but all I could do was to cut the power.
I reapired permissions, moved catalogues to mac in 'pictures' and tried again.
I managed to open a catalogue, looked at a picture & then it crashed.
Optimized LR2 catalogues etc
reinstalled etc.
Now all I get is the LR3 logo and system crashes immediately.
Help - rather annoyed that I've just spent £78 for this.
The LR2 upgrade when I had it worked flawlessly.
Others seem to be having the crash further on - I can't even look at catalogues now.
Update: just repaired permissions and am getting:
ACL: found but not expected for many strings - dont know hat t means

Answer 1

I have also had this thread on Canon Forum and Lightroom Forum where I followed the path you suggested.
After various solutions including setting up a clean user account, it was suggested (on Lighroom forum) that I try and open LR3 in 32 bit - which I did and it worked.
I then looked at LR2 which had been working as I noticed a comment that it too was 64 bit - but I had been running it in 32 bit.
I changed LR2 to 64 bit and it also crashed in exactly the same way - so It appears that for LR2 , on installing, the default was 32 bit and for LR3 the default was 64 bit.
Looking on the system requirements for LR3 it says that I ned at least 1G ram - but I have 3G! However, other comments are that LR3 needs 4G to work properly.
Many thanks for all your help - and I hope this thread helps others.
The error message below was what I kept getting every time I tried to load LR3 - the screen froze and I could do nothing but cut the power.