weird -im running on xp and has no issues at all.
Are your xp updated clean and not pirated?
You can check and try a 'repair xp' -also, You may like to search this forum for 'XP' and see if any of the previous problems other has had, can aid you in finding the reason.
Of the top -Do you run some icq /msn /skype at simultanous with lmms?
then, Shut down all other proggies and try again