I have just installed Yahoo! Messenger on Ubuntu 7.04.
At first when I installed it, there were some dependencies that had to install first,
which are libssl0.9.6 and xlibs (>>3.3.6), whereas the currently installed version in my Ubuntu was libssl0.9.7 and xlibs1.x.x (I don't really remember it).
Then I installed the libssl0.9.6 and xlibs4.x.x as requested, and
my YM installed succesfully, but it just couldn't connect. I don't why. The status is always "connecting" for the whole time until I closed it again. But that's not the point.
Everything was normal until the end of that day, but the next day I logged in to my Ubuntu again...
I couldn't click anything on my desktop!!
What should I do? Is there any better solution than re-install my Ubuntu again?
Thanks in advance.

I take it that your mouse pointer moves, but nothing happens when you try to click on an object.... is this just on the desktop or on the taskbar aswell?

what happens if you "right click" on something on the desktop?

I'm thinking you might have more of a hardware probelem, such as a faulty, or bad connection to the mouse.

it was both on the desktop and the taskbar. it was basically everywhere!
when i right-clicked the mouse (everywhere), it zoomed-out the desktop (i activated the desktop animation),
and if i held on the right click, the desktop animated like it followed wherever i move my mouse to.
that was the only thing that happened.
but the left-click didn't work everywhere.
i don't think it was because the bad connection to the mouse, because i've tried that with the touchpad on my laptop as well but it still didn't work.
besides, my mouse still work well on windows.

Ok, that rules out hardware faults then.

Your problem may well be with the packages you installed. I cant quite recall what package manager Ubuntu uses.... a quick google search suggests it is dpkg.
I would try to boot into a terminal/console, if Ubuntu gives you that option (if not, ditch it and get a real distro of Linux).
Try dppk --help and look for an option to remove the packages you installed... this might not fix your problem though, as the dependancies of YM could possibly be essential modules.
Failing that, your easiest solution in my opinion is to reinstall, but keep your home partition (do not format it, and tell the installer to mount it as /home) - This way most of your program settings, desktop and other stuff will still be there when the system is running again.
Once again, in this scenario, if Ubuntu doesnt let you keep your home partition, get rid of it and use a renowned distro - SUSE, Fedora, or if you really like Ubuntu, use Debian instead - thats what Ubuntu is based on anyhow.

Well yeah,
I finally came to conclussion that there's no better way than reinstall it, bcoz I couldn't even get into the console.
I just haven't decided whether i'd still use Ubuntu or try another distro.
Thanks a lot for the help anyway