Cannot update or install programs through synaptic

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http://www.linuxforums.org – Hi , I am new to Ubuntu and i have recently installed karmic on my laptop. Initially i was not able to connect to net through firefox but after googling around i found i needed to disable ipv6 in firefox. It is working well now. I also had disabled ipv6 globally as suggested by some threads. However i am not able to install any packages or programs through synaptic or software centre. Synaptic does not reload the packages and gives the following message after waiting to download files for a long time untu/dists/karmic/Release.gpg. Failed to fetch .,://archive.ubuntu./ubuntu/dists/karmic/main/i18n/Translation-en_IN.bz2. Failed to fetch .,://archive.ubuntu./ubuntu/dists/karmic/restricted/i18n/Translation-en_IN.bz2. Failed to fetch .,://archive.ubuntu./ubuntu/dists/karmic/universe/i18n/Translation-en_IN.bz2. (The above links are modified as i am not allowed to post links unless i am 15 posts) If i try to download after selecting it just wait for a long time showing 'downloading file 1 of total ( some number )' and stops to give the following message (for example ) W: Failed to fetch .,://archive.ubuntu./ubuntu/pool/main/p/pmake/pmake_1.111-1build2_i386.deb. Some suggested to change the server and i did this several times but no use. I dont think it is a problem with the server as i am able to download the deb files if i paste the link in firefox. I even installed skype like this by individually downloading the packages but i this is very tiresome as you can imagine. Skype is working perfectly. I am also not able to update. Update manager gives the list of packages to download but does not download them. I have gone back to ipv6 ( not in firefox ) thinking this may solve it. I am able to install all programs without program in my father's comp which uses same server (main server) but different internet connection. When i was using ubuntu9.04 there was no problem. I asked in the official ubuntu forum but no help. I use acer laptop 1641Nwlmi M725 pentium intel 1.6 GHz, 512 MB ram, intel graphics media accelerator 900 Can any body please help ? (HowTos)