synths play notes by themselves

  When I insert a soth synth and play it through my midi keyboard - it has a mind of its own and plays random notes interupting what im trying to record. It does this no matter what synth I use ansd I cant stop it Im not using an external sound card im just using the built in one My computer is a Aspire Revo Im using Sonar X1 Essential and my Midi keyboard is an old Yamaha SY35. It does the same when I pug in a Yamaha DX21 aswell so I can rule out the faulty keyboard ANy ideas how I can stop it doing this? Is it because my internal sound card is struggling? Im thinking of buying a Novation nio 2|4 external sound card - would this help? Shall I buy a USB keyboard instead of a MIDI output on the SY35?