I found this in a few different places around the 'net - hopefully it's helpful:

Add a Map Drive Button to the Toolbar

Do you want to quickly map a network drive, but can't find the toolbar button? If you map drives often, use one of these options to add a Map Drive button to the folder toolbar in My Computer.

Option One (Long Term Fix):

• Open My Computer, right-click the toolbar, then unlock the toolbars, if necessary.
• Right-click the toolbar again and click Customize.
• Under Available toolbar buttons, locate Map Drive, and drag it into the position you want on the right under Current toolbar buttons.
• Click Close, click OK, and then click OK again.

You now have a drive mapping buttons on your My Computer toolbar, so you can map drives from any folder window. To unmap drives, follow the above procedure, selecting Disconnect under Available toolbar buttons.

Option Two (Quick Fix):

• Right-click My Computer and choose Map Network Drive. If you place your My Computer icon directly on the desktop, you can make this move in only two clicks! Otherwise, you can right-click the My Computer icon in the Start Menu.