Wrong notes when playing MIDI files

Greetings everyone ! This is my 1st post. I am running a Dimension E521 desktop system. I am a musician & write music on my computer. I also frequently exchange MIDI files with other musicians. The problem I'm having when playing back MIDI files is: All notes are played back "flat" or in non-musical terms ->out of pitch Example -a D note plays back as a Db or C#. I was able to verify this with an electronic instrument tuner. This means that I cannot follow along with a musical instrument (piano, guitar etc) that is in perfect tune. I spent over an hour on the phone with a DELL help desk rep. We downloaded & installed new drivers for Codec353, Sigmatel + a different notation player. This made no difference. He suspects that it is a hardware problem. Really disapointed since I bought this machine to write music. Any ideas ? Marc