what computer r u using? ram? hdd rpm? sometimes, programs crash if it can't handle the material. i wouldn't be surprise if the hi-def material is a bit steep for your computer. the only solution would be to upgrade your setup. buying imovie 09 will not solve this, though.
as someone else suggested, try converting your video files into apple intermediate codec using MPEG Streamclip, which is free and a great program. and see if this will work.
and if you wanna check out videos in all its hi-def glory - there is that users based website, called vimeo dot com.