I experienced the exact problem a few weeks ago with my wife's pc. Went into a perpetual check disk mode and would not boot up. This is also currently happening with another friend of mine's laptop. In my case, there were two drives in the system, the primary 6bg drive that came with the system and a secondary 80gb drive I installed several months ago.

Once I unplugged the second drive, the system was able to boot normally, I have a suspicion that the boot sector of the second drive is corrupt in some way.

In your case, if recovering the data on the drive is not an issue, I would consider reformating the hard drive and reinstalling doing a clean install of the operating system. I know it's drastic, but it will probably be the quickest way to get things up and running again.

My buddy is a real "Geek" so as soon as he figures out what is wrong I will post it here as an FYI.