04A13

Can anyone help? I have Sibelius, but it keeps lagging! When I click play (or press the spacebar), it stops responding for about 5 seconds, then bursts out playing about 5 bars into it. this si really annoying! Also, if I'm playing something fast, it skips some notes. I use my Yamaha keyboard and connect it to my computer via MIDI to USB. I have Vista, 2 Gb of RAM and an AMD athlon 64 processor 3800+ 2.40GHz. Please can anyone tell me what is wrong and how can I fix it?? thanks