The following is from http://windowsxp.mvps.org/searchwindow.htm:
If you double-click a drive or folder, Search Companion may start and the drive or folder may not open. 'Search' will be the top most in the right-click context menu for a folder or drive. This usually happens if you've edited the File Folder or Drive via the Folder Options File Types dialog, in order to add a context-menu item such as "Command Prompt here", "Print Directory" feature for folders or drives.
References:
http://support.microsoft.com/?kbid=321379
http://support.microsoft.com/?kbid=321186
Quick fix
Click Start, Run and type this command:
regsvr32 /i shell32.dll
(Tip from David Candy)
To fix the problem manually, open Registry Editor and navigate to:
HKEY_CLASSES_ROOT\Directory\shell
-and-
HKEY_CLASSES_ROOT\Drive\shell
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In the right-pane, locate and click the (Default) value under the keys mentioned above
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Click Modify on the Edit menu.
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Type the word none in the Value data box, and then click OK.
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Exit Registry Editor.