Well i Can Tell you that the reason that you computer might be laggin up its most probably because of maplestory and its most likely ur graphics card that is freezing up ir computer and making it run slow and if u think that you might have a virus u can always try downloading a 90 trial of Norton Antivirus or McAfee
and ur modem might have been out of date driver or so and thats why ur computer needed to download additional files so its probably nothing. hope it helped