right click on "my computer" and select "Manage". Then go to "Disk Management", select the partition you want to delete, right click again and select "Delete".
Concerning the boot menu: right click again on "my computer" and select properties. Go to the tab "Advanced" en click in the "Startup and Recovery" section on the button "Settings". In the section "Startup Settings" you can select the default OS from the list. After hitting apply, you can click on the "Edit" button to edit the boot.ini file by yourself. Remove the line, which mention the XP Setup and save the file. Now you pc should startup XP by default, without bothering you with a menu asking which OS you want to start
.
Hope this helps!