When you're cruising the web, do those little pop-up ads, bother you? If so, one of the companies you have to thank is Gator Corp., an Internet advertising firm. It makes software that monitors what you do online and then displays those ads when you visit certain websites. Gator has cut deals with various software makers to have them include its software on their programs. As a result; millions of computer owners have Gator's software on their PCs.