Hello TheDean, if you download and then install a program called
VistaBootPRO. Once you have installed it, launch the program up and select the "Manage OS Entries" tab. In the "Select an entry to enable options below" box, there should be two options listed, "Microsoft Windows Vista" and "An Earlier version of Windows" which is Windows XP (I am not quite sure if it will be called that exactly, something along those lines though). Select the "An Earlier version of Windows" option and then on the right hand side click on the red X button. Now, when you restart your machine, it should, theoretically, boot straight into Windows Vista.
If the above fails, launch the VistaBootPro program again and select the "Manage OS Entries" tab completely remove all of, what is listed in the "Select an entry to enable options below" box, including Windows Vista. Then at the bottom, check the "Add New OS Entry" box, 3 options should appear. In the "OS Name" box, type Microsoft Windows Vista (Which ever name you would like to give it

). In the "OS Type" select the "Windows Vista" option and finally "OS Drive", select the appropriate drive letter where Windows Vista is installed. Then click on the "Apply Updates" button.
