I don't know what's really causing this, but u could try booting off ure rescue/install cdr/floppy, or boot into initlevel 1 by adding "single", "s", "rescue" or "init=/bin/sh" at the boot prompt.
try to filter tru the /var/log/messages and dmesg to see where if possibly fails. If the system or application logs don't give info, u can try to shut down as many services possible before trying to boot in ure normal initlevel.
If uve got a graphical login, then its initlevel 5, if not its usually initlevel 3. look for a corresponding directory in /etc, could be /etc/rc.d/rc(number).d/. now change for any service that is *not* system critical like sendmail, dhcp, rpc, bind, httpd, apache, samba, ftpd, routed, snmpd or sound the capital S(start) for a K(kill). like change S10sendmail to K10sendmail.
Restart, and if that works ok, try to start each service manually to see what gives.