Does popping up the main menu by pressing the 3rd mouse button cause
the system lock up?
This is a known problem with Ubuntu 10.04 LTS lucid and Xorg 1.7.6
which is discussed in more detail here:
http://groups.google.com/group/baudline/t/5b2574052f2e0a1c?hl=en_US
The solution is to upgrade Xorg to version 1.8.# or downgrade to Xorg
version 1.6.# which is used by Ubuntu 9.10 karmic. Whatever you do,
avoid the Xorg 1.7.# version. What makes this issue extremely
troubling is that Canonical (Ubuntu) was aware of this problem months
before 10.04 shipped. The reason why Ubuntu chose to ship their LTS
(long term support) Linux release with such a broken X-Server is
beyond explanation.