Problem: can't log in/su with any non-root user

["Jeremy Morgan" <porp@altavista.net>]

I'm not sure if this has been posted before (I've looked but can't find
anything.)  I recently reinstalled debian because the only user that my
system could use was root.  When I tried to su to any user except root I
would get error messages:

    su: cannot run /bin/bash: Permission denied

When I tried to login as another user:

    Unable to cd to "/home/porp"

Now this creates many problems except the obvious one (I have to use root
always): no daemons or programs can run as another user--exim (mail), the
cron.daily find (nobody), or any others.  Before I continue, I'll list the
permissions of certain files:

-rwxr-xr-x   1 root     root       364336 Jul 25 02:45 /bin/bash
-rwsr-xr-x   1 root     root         9512 Jun  2 09:59 /bin/su

drwxr-sr-x   2 porp     porp         1024 Jul 24 14:49 porp
drwxr-xr-x   2 root     root         2048 Jul 25 02:39 bin
drwxrwsr-x   4 root     staff        1024 Jul 24 22:14 home

I've been on irc.debian.org and have asked many people to help me discover
the problem but no one has been of much help.  Nevertheless, I decided to
reinstall.  I figured doing that would fix all problems.  And it did.  I
could login as any one, exim would work, and I was happy.  But as soon as I
upgraded to potato (I like many of the packages in unstable), every thing
broke again.  Now I'm sure this can't be a widespread problem because I
would have found solutions.  But I cannot figure out what the problem is.
It can't be the permissions on bash because if I change the user's shell in
/etc/passwd, I get the same error, just a different shell is listed.  It has
to be with some potato packages, either one or a combination of some are
conflicting or broken.  But then again I do not know because many people use
potato.  I just know that I'm not smart enough to figure it out, and my
system is broken. I really do hope you guys can help.