Keep tapping F8 just before you would normally get the Windows loading screen to bring up the boot menu. Assuming you have SP2 installed you should have an option that says 'disable automatic restart on boot failure' or something like that.
Select it and it should start booting as normal, when it gets to the point where it would restart it should stop on a blue screen with an error on. If it says 'unmountable boot volume' anywhere on it it's a 2 minute fix that doesn't need a full repair install doing.
Let us know what the stop screen says
