Ubuntu 10.04 LTS size-mismatch error during package installation

Hi,
I have installed ubuntu10.04 LTS 64-bit version two weeks back and tried to install packages using apt. Whenever I tried, I got the same error saying size mismatch. I am connected to internet via college intranet and have configured proxy settings in Preference>Network Proxy. Please help.

I was trying to install openfoam.
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The detailed error info:
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Failed to fetch http://archive.ubuntu.com/ubuntu/poo....6-1_amd64.deb Size mismatch
Failed to fetch http://archive.ubuntu.com/ubuntu/poo....5-1_amd64.deb Size mismatch
Failed to fetch http://archive.ubuntu.com/ubuntu/poo....3-1_amd64.deb Size mismatch
Failed to fetch http://archive.ubuntu.com/ubuntu/poo..._2.0-2_all.deb Size mismatch
.
.
.
Failed to fetch http://archive.ubuntu.com/ubuntu/poo....1-1_amd64.deb Size mismatch
Failed to fetch http://archive.ubuntu.com/ubuntu/poo....7-1_amd64.deb Size mismatch
Failed to fetch http://archive.ubuntu.com/ubuntu/poo...ntu1_amd64.deb Size mismatch
Failed to fetch http://archive.ubuntu.com/ubuntu/poo....1-2_amd64.deb Size mismatch
Failed to fetch http://archive.ubuntu.com/ubuntu/poo...sg-3_amd64.deb Size mismatch
Failed to fetch http://archive.ubuntu.com/ubuntu/poo...u7.1_amd64.deb Size mismatch
E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?
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