Are you trying to run the latest SL 3.2.8 version? When I upgraded to that viewer, I has login issues and if I got logged. in, I could not get anything to rez and had to log back out or I crashed. It was worse if I had voice chat enabled. So I wiped it off my laptop entirely. Then I installed a fairly old version of phoenix viewer (available in the release notes section of the phoenix wiki). I ran that version and it was ok, except mesh objects did not display correctly. So I wiped that one off and installed the 1.6.0.0 version of Phoenix which includes mesh display and it all runs fine. I've read on here, and I saw it firsthand myself that the newest SL viewer totally eats up the CPU. That may be part of the problem, but there seems to be a real issue enabling the voice chat in the new SL viewer, at least for me. So I recommend you try what I did. Also I found the ZEN viewer works well and takes up a lot less of your resources, just doesn't have as many bells and whistles as the SL or Phoenix viewer.
Hope this helps.