You may have to work very hard since causes are many. From pests, virus, trojans, Norton Antivus or other.
Use all the tools and more.
i have a 733mhz HP PC with windows ME, 128m RAM, and about 10G free on the hard drive
I can't play back any video format (mpeg, quicktime, divx, etc) smoothly...the video becomes incredibly choppy, and it is impossible to view the video at anything higher than 80% size. i have tried several different programs to play the files, and the problem seems to affect all of them. i'm pretty sure that this condition wasn't always there...it only showed up after my computer had been on a network. however, now, it isn't networked, and i'm not sure how to fix this..