SPM has a DICOM conversion utility that works with many different variations of the DICOM format. It is accessible via the button "DICOM Import" that will redirect you to the batch interface:
SPM -> Util -> DICOM Import
You will then have access to a number of options such as the choice of single file ('.nii') or dual files ('.img/.hdr') NIfTI output format.
If you encounter problems with your DICOM data, please email the SPM mailing list with details about your scanner and acquisition settings (an example of a DICOM file might be useful). This will contribute to make the DICOM conversion utility even more robust.