[CentOS] System Recovery

I hosed my main desktop pretty badly a couple days ago, have reconciled 
myself to bad news and would appreciate some advice.  (CentOS 5.3 fully 
updated but now backed down from kernel  2.6.18-128.7.1.el5 to  
2.6.18-128.4.1.el5 just to try that.)

The whole debacle began when I was investigating my formerly-weekly 
backup script to run to completion.  The backup is kept on an external 
USB-connected drive that is mounted and unmounted by the backup script. 
I have manually mounted it to retrieve a file but it has been months 
since I did that.

I deleted one copy of the backup, as I've done in the past and made a 
new crontab entry to try the backup again 2 or 3 minutes later. It was 
still running several hours later, which couldn't possibly be right.  
Next, I tried clearing the backup on the backup drive and manually 
copying directories to it.  That didn't work too well, either.  At some 
point, I tried to send an email using SeaMonkey and couldn't, because it 
was unable "to write a temporary copy".  I quickly found that that 
wasn't all it couldn't write, too.

I tried restarting KDE, which got nowhere.  Almost everything  worked 
O.K. from the command line, the most obvious exception being that I was 
unable to read any man pages as a non-privileged user until after I had 
accessed that man page as root.  I did an rpm -Va  with output 
redirected to a USB memory dongle.  There is stuff missing and mangled 
but nothing that raised my suspicion.

Finally, here is my selinux configuration.
[rj at mavis ~]$ ls -l /etc/selinux/config
-rw-r--r-- 1 root root 511 Dec  5  2007 /etc/selinux/config
[rj at mavis ~]$ cat /etc/selinux/config
# This file controls the state of SELinux on the system.
# SELINUX= can take one of these three values:
#       enforcing - SELinux security policy is enforced.
#       permissive - SELinux prints warnings instead of enforcing.
#       disabled - SELinux is fully disabled.
SELINUX=disabled
# SELINUXTYPE= type of policy in use. Possible values are:
#       targeted - Only targeted network daemons are protected.
#       strict - Full SELinux protection.
SELINUXTYPE=targeted
# SETLOCALDEFS= Check local definition changes
SETLOCALDEFS=0
[rj at mavis ~]$    

Does anyone have any ideas how I might attack this thing?  An epiphany 
has not been forthcoming.  :-(