Hello DaveTaylor.
If reboot doesn't work (good point, angryman) , you should try booting with your installation DVD and click on "Repair Computer".
If no automatic repair does any trick, then the solution is:
Click repair computer (still on installation dvd) and press next in all screens until you get to the screen with the options for Startup Repair, System Restore, Command Prompt etc. Chose Command Prompt.
At command prompt, type (exactly like below):
bcdedit /set {default} detecthal yes
and press enter.
Reboot and it should do the trick.