I just discovered this post from Daniel McPherson. It's very useful to know how to change the Category navigation bar on the home page of an SPS portal. I advice you to read his post, event if I give you the tip here : he explains why NOT delete the native topics area.

1. Open :\Program Files\Common Files\Microsoft Shared\web server extensions\60\TEMPLATE\\SPS\default.aspx
2. Search for the following string:   <SPSWC:CategoryNavigationWebPart runat="server" id="VerticalNavBar" DisplayStyle="VerticalOneLayer" /> 
3. Replace it with the following string:   <SPSWC:CategoryNavigationWebPart runat="server" id="NewVerticalNavBar" DisplayStyle="VerticalOneLayer" CurrentCategoryID="<INSERT CAT ID HERE>"/>
4. Where is equal to the Category ID of the area whose children you wish to display as navigation. To get the Cat ID: Browse to an area, then click on "Change Settings" and copy the string that appears after "CatID=" in the URL. "