Earlier I had an issue with yum not installing 7zip (or indeed anything) on a CentOS machine. I was trying to run;

sudo yum install p7zip

The fix (provided by @braziel) was to run the following 4 lines of code, which seem to update yum effectively.

wget http://packages.sw.be/rpmforge-release/rpmforge-release-0.5.2-2.el5.rf.i386.rpm
rpm --import http://apt.sw.be/RPM-GPG-KEY.dag.txt
rpm -K rpmforge-release-0.5.2-2.el5.rf.*.rpm
rpm -i rpmforge-release-0.5.2-2.el5.rf.*.rpm

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