The problem: I can open .doc and .sxw files from a Windows partition in the OpenOffice (in a xfce4 environment) but they open in a read-only mode. I don't have any idea about how to change that in order to work with the documents in the usual manner (i.e, without having to make copies of them in the Linux partition). I thought that this was a problem with permissions, but it doesn't seem to be so, since I've chmoded the files to 775 and my ordinary user is in the "wheel" group. I've tried to check if the fstab wasn't mounting the partition in 'ro' mode. It was set to 'default', but to eliminate doubts I've set it to 'user,rw'. As a final move I've tried to 'chown' the files to my ordinary user name (they were owned by root), but then it has appeared a new problem: the files could not be 'chowned' and I've had a kind of 'permission denied' error. How can I fix this?

NTFS partitions are normally mounted read-only, period. The newest kernels have kind of a hack-job way to change existing files, but it is sketchy at best. You'll have to either copy the files onto the linux partition, or use FAT32 partitions for things of this nature.

Your file permissions (i.e. the chmodding) won't have any effect because windows partitions and the FAT and NTFS filesystems don't provide the ability for permissions like linux does. If your windows drive is NTFS then chances are the NTFS kernel module you're using doesn't provide write capability. NTFS is crappy anyway.