7 hours!?
ok, iMovie doesn't open mpeg, avi, mp4 or mov files. only DV files. You can export those formats to DV with Quicktime Pro. There is also a free applescript which does this somewhere on Apple's website. You may have problems with the audio, though, since QT doesn't export mpeg audio. You can extract the audio with bbDemux, and encode it to mp3 or aiff with itunes if you change the name from something.m1a to something.mp2.
There's probably a way to export to dv on your PC too, but I don't know what it is.
You should also know that iDVD can only burn up to 90 minutes on one dvd, or 60 minutes at full quality. DVD Studio Pro ($999) can squeeze up to 6 hours on one disc (I think).