Can't connect (2 dbs) or login (2 others)

We have a cluster with four nodes, each running a postgres 7.4.8
database. Due to a large amount of pilot error and possibly hardware
problems (still trying to get to the bottom of it all), two of the
databases won't start, and I can't login to two others, with any
registered user.

Two of the nodes have logs that look like this:

2007-09-22 07:06:05 [3055] LOG: could not create IPv6 socket: Address
family not supported by protocol
2007-09-22 07:06:05 [3060] LOG: database system shutdown was
interrupted at 2007-09-22 06:56:27 PDT
2007-09-22 07:06:05 [3060] LOG: could not open file
"/var/lib/pgsql/data/pg_xlog/0000000000000000" (log file 0, segment
0): No such file or directory
2007-09-22 07:06:05 [3060] LOG: invalid primary checkpoint record
2007-09-22 07:06:05 [3060] LOG: could not open file
"/var/lib/pgsql/data/pg_xlog/0000000000000000" (log file 0, segment
0): No such file or directory
2007-09-22 07:06:05 [3060] LOG: invalid secondary checkpoint record
2007-09-22 07:06:05 [3060] PANIC: could not locate a valid checkpoint record
2007-09-22 07:06:05 [3055] LOG: startup process (PID 3060) was
terminated by signal 6
2007-09-22 07:06:05 [3055] LOG: aborting startup due to startup process failure

The other two look like this:

LOG: database system was shut down at 2007-09-22 05:52:26 PDT
LOG: checkpoint record is at 0/9B0B8C
LOG: redo record is at 0/9B0B8C; undo record is at 0/0; shutdown TRUE
LOG: next transaction ID: 536; next OID: 17142
LOG: database system is ready
FATAL: IDENT authentication failed for user "xxx"
FATAL: IDENT authentication failed for user "yyy"

Can someone tell what is going wrong and how I can recover?

Morris