[CentOS] Problems eith yum (wrong releasever)

After installing clean Centos 5.3 and updating with "yum update" yum starts
to fail. I get the following error message


YumRepo Error: All mirror URLs are not using ftp, http[s] or file.
 Eg. 7 is not a valid release or hasnt been released yet/
removing mirrorlist with no valid mirrors:
//var/cache/yum/base/mirrorlist.txt

It looks like the $releasever variable has value 7. Why does this happen and
how do I fix this?

Thanks,

Sami
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