What's wrong with my smb.conf and why can't Windows access the share?

That's really quite interesting. I'm using a lot of the same configuration options for my Samba34.

That being said, hit up man 5 smb.conf - this will list all the acceptable clauses you can use. Don't forget, also, to make sure each thing is in the correct section. For example, don't have server string = UBUNTU under [Myshare]. By convention, each section should start with a lower case letter. I'd expect Samba to be a little bit more robust, but perhaps it isn't being recognised as a share?

Also, I don't believe read_only and guest_ok are valid entries. The rest of them are valid, so they're most likely not being recognised as applying to any share; they're sitting loose and are hence being ignored.

Start with a light configuration and slowly work your way up. Comment out lines with # and add them back until you find out what's doing it. I don't think [Myshare] is the bit with the issues; I think you've used [global]-only fields under different shares or something like that. The man page should explain where each can be used.

WinXP will behave like that if it is able to connect to the share but the top level directory itself doesn't allow reading. It would be good to double-check the permissions are right. You can verify that the right user is being seen by the samba server by running smbstatus while the XP machine is connected to the share. Then verify that the specified Username can read the share's directory.

Not that this is the safest way to do it, but you could try something like this. Then, once you have it working, lock it down with tighter security:

security = share 
guest account = publicuser
passdb backend = tdbsam

then

[publicuser]
comment = publicuser Home Dir
path = /home/publicuser
public = yes
writable = yes
write list = +publicuser
browseable = yes
hosts allow = 192.168.10.10

and, in /etc/smbusers

# Unix_name = SMB_name1 SMB_name2 ...
# root = 
publicuser = Administrator smbguest pcguest guest
# nobody = guest pcguest smbguest

I know this thread was buried a long time ago but Google brought it up when I have an almost identical looking problem as the OP - my logfiles looked v v similar.

For me it turned out the the line endings in smb.conf were the problem.

Somehow (when I say somehow I really mean - Because of sloppy typing) I had lots of lines with a whitespace at the end and they were all getting ignored = didn't show up when I used the testparm command - might have been some quirk in copying them down from github. Cleaned up those whitespaces and the config worked a treat!