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I'm on 'Snow Leopard' OS X 10.6.8 and Safari 5.1.8, so these screenshots are from my computer.
The specific instructions for the 'Leopard' OS X 10.5.8 Safari will be similar, but not identical.
First, your 'Spotlight' Search window in the upper right corner of Safari probably has a downward-pointing arrow. Click it to temporarily select another search engine at the bottom.
To permanently set the search engine preference, open the Safari, Preferences, General tab, select Default Search Engine, and select Google.
Sounds like you might have installed a Bing add-in or Bing toolbar in Safari. Sorry, but I can't tell you how to uninstall that, since I've never used it or had to uninstall it.
In any case, you MIGHT be able to restore your Search toolbar this way:
(These are screen captures from Safari 5.1.9 from OS X 10.6.8, so yours will likely look different)
Try Safari, View, Customize Toolbar, then go to the bottom and use or drag the default set onto the toolbar
One other possibility is that you've acquired a malicious 'DNS changer' that redirects your internet traffic. Sorry, but I can't help you with that either.