Can't Edit Most Things !!HELP!!

In my admin, when I try to add or edit most things and click the save button I am returned to the main page and nothing is saved eg if I try to add a new category in Categories/Products nothing happens, if I try to edit my details in My Store nothing happens. I can, however, add a new product and edit an existing product in Categories/Products.

Please click Reply and answer all the questions about your Zencart installation.

v1.3.8
[2009-12-20 20:52:44] (Version Update 1.3.7->1.3.8)
PayPal IPN about 18 months ago
Site is not customized except for header & page color-done years ago
No display/layout problems
I've looked in the FAQ's and forums-nothing matches
Problem started 2 months ago. As far as I know neither my hosting company nor myself have done anything.
This store has been live since 2005

1.3.8 ...

Possible hack, maybe ... ?

1.3.8 is dangerous these days.

1. you really should update
2. What version of php is your server running
3. Do you have any admin mods