I'm running Ubuntu 10.04 on a desktop with an Edimax EW-7811UN wireless USB adapter. I just got the computer online earlier today. My internet source is tethered from my Android smartphone, so it can be a bit slow at times, but it works. Except when I try to get new software from the Ubuntu Software Center. Downloads reach 50%, then suddenly stop with a message telling me to check my internet connection, and this: "Failed to fetch http://security.ubuntu.com/ubuntu/pool/main/s/samba/winbind_3.4.7~dfsg-1ubuntu3.8_i386.deb 404 Not Found [IP: 91.189.92.200 80]"

iwconfig gives this reading:
Code:
jacob@jacob-desktop:~$ iwconfig
lo        no wireless extensions.

eth0      no wireless extensions.

wlan0     IEEE 802.11bg  ESSID:"Dr Dugong Loves You"  Nickname:"<WIFI@REALTEK>"
          Mode:Managed  Frequency:2.437 GHz  Access Point: 44:A7:CF:EC:49:F9   
          Bit Rate:48 Mb/s   Sensitivity:0/0  
          Retry:off   RTS thr:off   Fragment thr:off
          Power Management:off
          Link Quality=100/100  Signal level=-45 dBm  Noise level=0 dBm
          Rx invalid nwid:0  Rx invalid crypt:0  Rx invalid frag:0
          Tx excessive retries:0  Invalid misc:0   Missed beacon:0
And ifconfig gives this reading:

Code:
jacob@jacob-desktop:~$ ifconfig
eth0      Link encap:Ethernet  HWaddr 00:14:c2:0e:c8:30  
          UP BROADCAST MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)
          Interrupt:17 

lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:52 errors:0 dropped:0 overruns:0 frame:0
          TX packets:52 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:3776 (3.7 KB)  TX bytes:3776 (3.7 KB)

wlan0     Link encap:Ethernet  HWaddr 80:1f:02:94:34:07  
          inet addr:192.168.43.232  Bcast:192.168.43.255  Mask:255.255.255.0
          inet6 addr: fe80::821f:2ff:fe94:3407/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:76042 errors:0 dropped:33780 overruns:0 frame:0
          TX packets:66385 errors:0 dropped:6 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:102757341 (102.7 MB)  TX bytes:9122626 (9.1 MB)
I'm still new to Ubuntu, and so I have no idea why this dialog appears or what to do about it. Any help would be highly appreciated!