Ruach,
You have spyware installed on your computer. These are little programs that run on your computer and do two things (not including steal resources). They send reports about your computer behavior to their owners, and they pop up advertisements.
The safest way to get rid of these nasty programs is to run a program to remove them. I have had luck with the free version of Ad-ware from
www.lavasoft.com. There are several commericial programs from reputible companies around that claim to remove these programs.
You have these programs because someone ran a program that they downloaded off the net. The neat little programs that put the weather on your toolbar, or promise to increase your computer speed etc all install spyware on your computer.
It is best to simply avoid these insidious little "free" programs altogether.