This is probably a function of the configuration of your network share. I'm not sure there is a great way to configure this to behave as you want without more details on the permissions. However, using a Subversion repository over a file share is really not recommended. It creates possible issues with locks, mismatches of Subversion versions (if you want to use an older client with a format it doesn't understand) and of course the permissions issues you're having.
I'd suggest rather than using file:// pointed at a network share to host your repository on other machines that you setup a SVN server. Which server is right for you is hard to advise based on the information available, see the Choosing a Server section of the SVN Book.
That said the easiest method for you might be svnserve over ssh. Doing that on the Mac should be as simple as turning on Remote Login under the Sharing pane of System Preferences on the Mac and then using a svn+ssh URL when accessing the repository from other machines. Details on how to construct the URL can be found in the Tunneling over SSH section of the SVN Book. You can just use the username for your account on the Mac for the user and your password for the Mac.
You don't say what client you're using on Windows but TortoiseSVN has directions on setting up to use svnserve over svn.