First off what is your operating system. Do you run any anti virus software. Do you use malaware detceters(spybot search & destroy ) Do you leave your computer on all the time. Are you on BB or dial up.
Two things, is your windows 2000 up to date with the latest service pack, is Norton Antivirus up to date. How long has this been happening and have you changed any software that might be the cause of this. I have spoken with a few friends who run windows 2000 and they think it is software related.