Your PC is running fsck (filesystem check) which checks the integrity of
your drives in a manner very similar to chkdsk (except that it actually
works!). The system is set up to automatically run fsck every x times
the disk is mounted (I believe the default is every 30 mounts, but I
could be wrong) just to make sure that there is nothing wrong with
them. I would not advise turning this off - and don't know how to
anyway - as it is a valuable check on your system's health.

As to why pressing C does nothing, I'm at a loss. I occasionally do
this when I'm in a hurry and it's never failed to work for me

Try Ctrl/C..............................

look at this post for changing frequency of disk checks:

http://ubuntuforums.org/showthread.php?t=300477

It's a little outdated, but I'm pretty sure it is what I used when I changed
this in 9.04, and worked fine then.

Sorry, don't know about the "C" thing. Have you tried "Esc"?

Yes, the commands should work whichever version of Ubuntu you're using -
they're pretty fundamental Linux commands.

Thanks for the link, I had no idea this was a recurring thing.
It's like a punishment if you shut down your machine a lot. Turns a
90 second boot into a 20 minute one.

As a side note, I don't understand why it would be necessary
ever unless the filesystem was unmounted properly.

20 minutes? When my disk checker utility runs, it's like 1 minute, or maybe 2.
How many terrabytes you got over there?

2 80GB IDE drives, one master one slave.

Disks do go bad.

I'd much rather wait for a disk check once in a while and get some warning that
my disk is going bad than try to reboot and get nothing....

I guess I've been lucky. I haven't had one go bad in over 20
years now. I wouldn't base my faith in a check disk anyway,
that's why I have/do backups.