Every time I want to open Software Center to install something, my computer hangs before it starts. What could be the problem?

AND

When I try sudo apt-get update and type my password, in the end of that I get error:

W: GPG error: http://repo.steampowered.com precise InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY F24AEA9FB05498B7

EDIT

When I run software-center command from terminal, it does not give any errors, only thing it writes in output is:

Fontconfig warning: "/etc/fonts/conf.d/50-user.conf", line 9: reading configurations from ~/.fonts.conf is deprecated.
2012-12-23 15:55:18,554 - softwarecenter.ui.gtk3.app - INFO - setting up proxy 'None'
2012-12-23 15:55:19,000 - softwarecenter.db.database - INFO - open() database: path=None use_axi=True use_agent=True

and after that it stays that way...

I have a problem when I try to connect my guitar to my computer, via line input. Audacity works well, and I can record what I play on my guitar (selecting from Preferences from where to record to default:line0). But the problem is in Qjackctl : at first when I click on start it shows this error:

after that when I click on Cancel it gives this error:

This is the log from the messages window:

10:30:37.368 Patchbay deactivated.
10:30:37.416 Statistics reset.
10:30:37.459 ALSA connection change.
10:30:37.497 D-BUS: Service is available (org.jackaudio.service aka jackdbus).
Cannot connect to server socket err = No such file or directory
Cannot connect to server request channel
jack server is not running or cannot be started
10:30:37.512 ALSA connection graph change.
10:30:44.060 D-BUS: JACK server could not be started. Sorry
Cannot connect to server socket err = No such file or directory
Cannot connect to server request channel
jack server is not running or cannot be started
Sat Dec  1 10:30:43 2012: Starting jack server...
Sat Dec  1 10:30:43 2012: JACK server starting in realtime mode with priority 10
Sat Dec  1 10:30:44 2012: control device hw:0
Sat Dec  1 10:30:44 2012: control device hw:0
Sat Dec  1 10:30:44 2012: [1m[31mERROR: Failed to acquire device name : Audio0 error : Method "RequestRelease" with signature "i" on interface "org.freedesktop.ReserveDevice1" doesn't exist
[0m
Sat Dec  1 10:30:44 2012: [1m[31mERROR: Audio device hw:0 cannot be acquired...[0m
Sat Dec  1 10:30:44 2012: [1m[31mERROR: Cannot initialize driver[0m
Sat Dec  1 10:30:44 2012: [1m[31mERROR: JackServer::Open failed with -1[0m
Sat Dec  1 10:30:44 2012: [1m[31mERROR: Failed to open server[0m
Sat Dec  1 10:30:45 2012: Saving settings to "/home/gevorg/.config/jack/conf.xml" ...
10:31:11.438 Could not connect to JACK server as client. - Overall operation failed. - Unable to connect to server. Please check the messages window for more info.
Cannot connect to server socket err = No such file or directory
Cannot connect to server request channel
jack server is not running or cannot be started

Now I can't understand what my problem is.

I have got a new webcam: Logitech. But today when I have connected it to my pc (ubuntu 12.10) and tried to open with Cheese Webcam Booth I've got something like this:

p.s. works good with "Kamerka"

EDIT camera is logitech C910

How would I go about determining if Google Chrome is downloading something, and getting the "percent-done" value of that download, from the terminal using a Python, Perl, or Bash script?

In my Unity there is one problem:

This issue hapened when I changed my icon theme to Mac-Lion-Theme

I don't know when this appears and what is this:

then when I click on that icon its showing this:

What does my computer do when I click ▸ Shut Down? I mean, is it running a script? What is it running?

Possible Duplicate:
How can I hide directories without changing their names?

How can I hide a folder and only see it when Ctrl+H is pressed? I have checked in Properties and couldn't find it.

I am using 11.10 and have some problems with Update manager. When I opening Update manager it's saying that I have some updates to install.

When I press button Install Updates, Update manager brings the following window:

My software sources settings is:

Can anyone say what is the problem? Thanks.

Is there any way to backup installed programs to DVD. For example when I will install new Ubuntu to install software from the disk and not from the Internet.

I have one problem. Just look at image:

So I want to remove one of Dev-C++ IDE and Microsoft Visual Studio Version Selector (I've tried to install Microsoft Visual Studio and it doesn't works,and one of the Dev-C++ IDE too). But I don't know how to change that list. I am using Ubuntu 11.10.

My LibreOffice is not showing images. They look like this:

Can someone help?

I have printer Canon LBP6000 and I tried to add printer from System Settings -> Printing. So I can add that printer but when I tried to print test page it doesn't printing. Can someone help me?

I downloaded Wireshark from the Ubuntu 11.10 software center and when I start it and want to select default interface for capture in the list of interfaces are blank. I can't select default interface. Please help me.

I downloaded VirtualBox from ubuntu software center and install there windows XP 32 bit. Now when I'm trying to install guest additions it's saying that guest addition is missing and is asking download it. When I press 'download' it's saying that it can't download because the link is missing.

When I install Windows 7 32bit in VirtualBox on Ubuntu 32bit I've got a problem.

The Windows 7 install can't make its resolution equal my notebook screen resolution of 1280x800.