1) It'll keep asking until you let it run to completion. (Your hard drive is marked "dirty". Checkdist will mark it "clean" on successful completion.)
2) Checkdisk can take days (the longest I've ever seen was a little over 7 days - on a 30GB drive - but I've heard of it taking longer.) It can look "frozen" for hours at a time. That's normal on a dying drive.
3) You unmarked the dirty bit. The drive is still dirty, Windows just doesn't know it. Your friend is right. One day the drive will just hard crash and you'll have to replace it. I hope all your important files are backed up.
4) Sleeping just works the drive harder (all of RAM has to be saved to sleep, then reread to wake up). It just isn't accessing the system area of the drive, which is probably where the problem is. Let Checkdisk complete and, if it's a minor problem, it'll be fixed. (I'd still clone the drive, "government format" it, toss it and replace it.