Can you provide some additional information as to where (and what format) did you get the original data? What did the names of the files look like? Generally a provider will de-multiplex the samples for you (using the illumina pipeline software). It is much simpler to do it that way.
You may be able to use the script from the Qiime package:
http://qiime.org/scripts/split_libraries_fastq.html to do the demultiplexing as suggested in this thread:
http://seqanswers.com/forums/showthread.php?t=24215