It\'s possible that you have bad memory or that you may have setup a MIDI feedback loop. Go through all your program settings one at a time. Sit back and think of anything you may have possibly changed since it worked correctly.
Cakewalk Home Studio should have a panic button to shut off stuck notes. It may look like an exclamation point icon. Sonar does have this feature.
I work with computers for a living and its not uncommon for them to come out of the box already broken.
do a search for memtest86 online and you can test your memory with it. It will create a bootable disk you can use. If it runs for an hour or so with no errors, you can pretty much assume your memory is good.
Okay, heres a fact: All Soundblaster cards are crap for making music! If you are serious about your music, you must invest in decent soundcard. Might I suggest the Terratec cards. I have truly excellent ASIO and WDM performance from them.
If you can afford them, the new EMU cards are terrific!