It sounds like /sbin/mingetty is broken, yet as init's job is to respawn the getty when it dies its just going around in a few quick loop, hence the 'respawning too fast' message. Booting from a straight forward boot disk won't help as it will bring the system up and then use the normal file systems from you hard disk(s) and hit the same proble with init/mingetty.
What you want to do is boot from something like a demo CD (there was a link recently on the forum, I think it mentioned 'demolinux' or something) so that the system comes up without touching the hard disks (it'll use filesystems on the CD). Once the system is up and you're logged on then you can mount your existing partitions somewhere (if its /dev/hdb for your root file system then you do a mkdir someDir; mount /dev/hdb /someDir
) and have a look at /someDir/sbin/mingetty to check if its there, and probably copy over it with a known good version.
However if it broke after you upgraded your C libs then you might have something a little more serious to sort out. The version of mingetty you have might not work with the new C libs, so you may need to get find a version of mingetty that works with the new ones. Either that of the libs you've installed are broken, in which case I think you'll need to do the same procedure as above (mounting from a demo CD) and replace your C libs with know good versions.
If you post exactly what upgrade you did (versions etc) then someone on the forum may have done the same upgrade in the past and be able to sort you out.
Not a nice one to fix really
Cheers, good luck