I have not seen many of these problems in recent months, but I have been collecting reports of solutions. Here is what I have gathered.
Turn off automatic download/install. (Preferences--Setup window).
Download manually and put the link to the set-up file on your desktop.
Reboot your system.
Install the viewer before you open any other program.
- Defrag your hard drive. Harmless. If it takes a long time, you needed it anyway, even it if does not fix the problem. - Make a new avatar. Your main avatar may have a bad attachment. If you can log in with a new avatar, you may be able to log in with your main avatar in a quiet sim (Pooly), especially at low load time, like early morning.
WiFi: If you are using WiFi and have problems, try a wired connection. WiFi drops packets. Your browser does a better job of managing dropped packets than does the Second Life viewer.
Router. This may be a router problem. Don't be misled by the fact that your browser works fine. Second Life uses additional ports. These ports can have problems without affecting browsers.
Reset your router
Disconnect your router from power for a minute or longer. Then reconnect it. This will reset your router. Test to see if the problem is fixed.
Test for defective router
Be sure your computer firewall is on, then bypass your router and connect your computer directly to your modem via physical Ethernet cable. If that fixes the problem, you probably need a new router.
If you have a Netgear router, there has been a known problem. It may have been fixed in current models, but see the note below
Just to let you know: I hv a netgear DNG1000 and had massive problems staying connected for more than2min. then i found this post (in italian) http://www.secondlifeitalia.com/community/viewtopic.php?f=4&t=27240#p260217 which suggests to use PPPoA/LLC instead of PPPoA/VC. problem solved!!
... Seems to be related to a DNS issue. By adding two backup google DNS entries [22.214.171.124 & 126.96.36.199] I was able to launch and run 3.0 without any apparent issues, although I will continue to monitor the result.