268420 joomla expert - updating issue

I have a joomla site - 1.5.7 and tried to update it myself and screwed up access to certain parts of the admin section like the install/uninstall area. all I did was update the files in the update from 1.5.7 to 1.5.8 Can someone help me correct this problem so that the admin section is fully working with no 500 errors when I click on them (like the install/uninstall plugin section. Anyone providing previous joomla websites and work examples of joomla will be considered the most qualified. So email me your examples and time your worked with joomla and what you see the resolution to my problem is? Bid accordingly to solve this issue - I am looking for on-going help since I am launching a network of site in joomla that new updates and plugin installs.