CRCDISK.SYS identifies itself as a "Disk Block Verification Filter Driver" and was written by Microsoft. Presumably, it does some kind of CRC checking on data being read off the disk.
With the volume of problems that I'm seeing being reported "about it" and the lack of official knowledge base articles about it, I have two suppositions:
It just happens to be the last driver that loads while the text-mode driver list is displayed on a "safe mode" boot, so people blame it even though a later part of the boot porcess is hanging.
It really does hang the boot process because something is going wrong.
Both of these are just suppositions. I'd love to hear from someone who actually knows. Reading the various "fixes" that I've seen posted in forums for "freezes at CRCDISK.SYS" and such, all I find is a lot of "witch doctor" superstitious stuff. I'm not finding where people have put together a reproducible error and resolution case. To that end, I expect that some part of the root causes probably is a transient hardware issue with some people. Given that a wide variety of computers have been reported as having problems "with CRCDISK.SYS", it does also seem likely that some later problem in the boot process could be hanging the machine up while poor CRCDISK.SYS just happens to have its name displayed on the screen and, thus, ends up being the object of peoples ire.
If you haven't booted the W2K8 installation CD in repair mode yet, I would. I'd run CHKDSK against your system volume from there and see if that improves behaviour any.