Midi Out of Tune on Dell E510 Sigmatel Windows XP Media Center Ed.

After testing various programs I have found that my midi output it out of tune. Everything is in relative tune on the system, but C from my midi output is not actually C when tested against external tuners and pianos. I've tried updating the audio driver and chipset to no avail. I have an E510 with Sigmatel audio and midi is based on the Microsoft GS Wavetable SW Synth, running windows XP Media Center Ed. I have tested Audio CDs through the CD drive and found that their playback is accurate to actual pitch. It is just midi that sits about 40 cents flat of actual pitch. Because I record audio and multi-track that with midi tracks, I need the two to be in tune to each other. What can I do to fix this? Any help would be massively appreciated (go step by step on instructions, I'm far from a computer wiz). Matt