From http://www.fjsmjs.com/IE/notoolbaar.htm

Try pressing F11.  If that doesn't work read on.

From Doug Knox:

Close any open Internet Explorer and Windows Explorer windows.

Click Start, Run and enter REGEDIT

Go to:

HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Toolbar

There are 3 sub keys:  Explorer, ShellBrowser, WebBrowser'

In each of these sub-keys is the entry for ITBarLayout

For IE Toolbars:  The WebBrowser Sub-key directly affects IE.
For Windows Explorer Toolbars: The Explorer Sub-key directly affects
Explorer.
The ShellBrowser sub-key may affect IE as well.  Its effect on IE and
Windows Explorer is unknown at this time, so if the first option doesn't
work, delete the ITBarLayout value here as well.

Choose the relavent sub-key and right click where it says ITBarLayout and
select Delete.

The deleted value will automatically be recreated for you.

You may want to right click on the main key (above) and select Export.  This
will allow you to backup the contents of the whole key, including subkeys.

Or try this utility, which does the same thing:
Toolbars missing in Windows Explorer or Internet Explorer
http://www.kellys-korner-xp.com/ToolbarRepair.Exe