I don't know the "why", but you may want to try the following:
Run this fix from Microsoft: http://support.microsoft.com/mats/Program_Install_and_Uninstall
Uninstall all traces of any versions of Reader using Add/Remove Programs
If you want to reinstall Reader, download its full installer from http://get.adobe.com/reader/enterprise/
If Vikram's suggestion solved your problem, could you mark this question as answered? And if mine didn't, could you please tell us what happened when you tried?
Also, it would perhaps be wise to remove all personal information frtom your last message. I am answering your first one because this way you can remove it yourself if you want to.