Well, it somewhat depends on the version you are running becuase it is a CSS thing and they have change the CSS slightly over time.
However, if you look at your customer/home.tpl file, somewhere near the bottom (around line 56) you will see the code:
Code:
<td width=20></td>
<td width=150 valign=top>
{ include file="customer/menu_cart.tpl" }
Change this to:
Code:
<td width=20></td>
<td class=VertMenuBox width=150 valign=top>
{ include file="customer/menu_cart.tpl" }
The addition of "class=VertMenuBox " is assuming that is the class being used on the left side. This is easily found out by looking near at the top of the file.