I installed tortoiseSVN and was able to do a checkout of the dirs/files that are already in the repository (I don't need to authenticate for that)
When I try to commit changes I get the following error:

Server sent unexpected return value (403 Forbidden) in response to CHECKOUT

I am never asked to enter my user/pass for authentication.

Tried googling it a bit, and found various mentions of this, but no definite answer.
I talked with the people in charge of the SVN server on our campus, and they claim that everything is OK on the server side...

Any help would be greatly appreciated :)