I never used "tortoise", but when I have big problems with Subversion I do that:

First, I copy only (and only) the files I modified to a directory out of the local tree. Then, I delete the whole local tree. After that, I checkout for a new local copy. Finally, I copy the previously copied files to the new local tree.

That solved almost all (few) big problems I ever had with SVN. If this doesn't solves your problem, may be there's a conflictive change somewhere or a corrupt file. Try to get the differences between the local copy and the server's one. In console use "svn diff <file>". I use KDEsvn some times because allows to see the differences of each file in a very useful way. KDEsvn is a great client and should run on Windows too (Kompare also recommendable).