Ive got the longhorn cd copied to my hard drive so I cant set my pc to boot from cd, so is there anyway to start longhorn setup in dos mode? cos i need to format my pc but dont want to risk it until I know if it will install in dos mode or not.

From the installation disk. That's the normal accepted method. Why would you attempt what might be a difficult or impossible task by installed it the non-accepted method when it's so easy to do it the right way first?

If i had the CD i wouldnt be asking. Its a long story but to get to the point, I dont have the CD. I do however have it on my hd. Thats my problem.

LongHorn dos install will be very hard as longhorn uses ntfs file system and dos don't recognise ntfs
also longhorn comes on a bootable dvd rom so you need to burn a dvd image to get that working mate
i have installed longhorn on my virtual pc and i cant see what all the fuss is about stick with xp at least that is stable longhorn that is out at the moment is a trial 120 days

I am testing the new Vista Beta, and right now there aren't a whole lot of options or drivers out there to get creative. You do need to burnit as an ISO file to DVD, and then boot up your PC with the DVD set as 1st bootable device in your BIOS. Make sure you uninstall any anti-virus software you have first. The new beta will NOT install with an AV program running,. even in stealth mode. At least that's been my experience. Stick w/ XP Home SP1 as the best stable OS right now.