It is apparently a very bad idea to run fsck on a mounted filesystem, even via single user mode. There are two good options for running it:
Boot from a CD with CentOS on it, and run it from the CD. Then none of your filesystems are mounted.
To do this, boot into Linux Rescue Mode:
boot: linux rescue
DO NOT mount your filesystems when it asks you if you want to. As noted above, you can't run fsck on a mounted filesystem.
Once you're in rescue mode, run fsck on the correct partition:
fsck -f /dev/sda1
If you've used the Linux LVM to create your partitions, you'll need to make those accessible before you run fsck:
lvm pvscan lvm vgscan lvm lvchange -ay VolGroup00 lvm lvscan
Then you can run fsck:
fsck -f /dev/VolGroup00/LogVol00
If you type
bash# shutdown -Fr now
then CentOS will reboot and do a forced fsck, which will be done before the filesystems are mounted.