have you tried logging in with another account to see if it might be profile corruption?
If using Windows XP you can use the F8 key to boot to Safe Mode and log in with the ADMINISTRATOR account.
If using Vista or Windows 7 you first need to activate the hidden ADMINISTRATOR account. Since you are able to use Task Manager, use it to browse to your Windows/system32 folder and locate CMD.EXE then right click it and run as administrator and type in this command:
net user administrator /active:yes then press the enter/return key
Be sure you type it exactly like that with the spaces and when you re-boot you will have Administrator as one of the accounts you can choose to boot the computer. If you leave it enabled give it a password you will not forget or you can disable it again with this command net user administrator /active:no
You might also try using System Restore to a restore point prior to the installation of Avast. You can do this either by using your Task Manager to browse to windows\system32\restore\rstrui.exe and running it or from Safe Mode with a command prompt.
So after downloading avast (from CNET) it told me to restart my computer after updating it. Now when I try to log on, my desktop is black and my start bar is gone. I can still run the task manager with ctr-alt-del and from there run programs but I can't even bring up the start menu if I press the windows button. Anyone know what's going on?