Why SVN makes read-only my files after check out or update?

Working with an standard configuration of the server, without any lock, my files are read-only after every check out or update to the working copy. I have to set them to be not read-only to continue working, but besides this shouldn't be happening, it's uncomfortable.

Anybody knows how to avoid this behavior?

EDIT

Answers to comments:

• Platform: Windows 32 bits binding, without Apache + TortoisseSvn.
• The files are not marked as read only on the first checked in to the repository (of course!, if they were this question would be silly)
• I don't think that it would be incompatibility, because it was working fine until it wasn't...
• The svn:needs-lock property is not set on the files.
• I'm using a local NTFS file system on WinXP, in an Active Directory environment (if that matter).

By the way, svn+tortoissesvn was working fine in the be开发者_运维技巧ginning, but at some point it stopped to do that.

Is the svn:needs-lock property set on the files in question?

http://svnbook.red-bean.com/en/1.5/svn.advanced.locking.html

http://svnbook.red-bean.com/en/1.5/svn.ref.properties.html

UPDATE

AFAIK, SVN will only set files as read-only if svn:needs-lock is set. If this property truly is not set, then something else on your system may be messing with the files. You could use Process Monitor to watch the files and see who changes them.