In Windows Vista, you can bring it back by doing the following:

1. Right click on the empty space on the Desktop and click Personalize.
2. Select Change Desktop Icons.
3. Check Recycle Bin.

and you are done.

The steps for Windows XP are a little more technical and involve registry manipulations which is not recommended unless you are an advanced user. Luckily, Microsoft has a tool that can take care of it automatically.

Link to Microsoft’s Fix it for me tool

1. Download the following zip file:

    For Windows XP – show_desktop_xp.zip
    For Windows Vista – show_desktop_vista.zip

2. Extract the contents of the zip file. It should be one file named Show Desktop.

3. Move that file to where ever you want to put the Show Desktop functionality. To put it back to the Quick Launch menu, copy the file to the following location:

    For Windows XP:
    C:\Documents and Settings\UserName\Application Data\Microsoft\Internet Explorer\

    For Windows Vista:
    C:\Users\UserName\AppData\Roaming\Microsoft\Internet Explorer\

That’s it. The Show Desktop icon is back in the Quick Launch menu!!

Source: Microsoft Support

The above link for Microsoft’s support website also has a tool to create this automatically for Windows XP. Check out the Fix it for me section.