mazzy1026,
One of the first indications something is wrong with the computer is a lot of popups. A program like Google Toolbar will stop those popups and therefore not give you the early warning something is wrong.
Malware is:
Adware - Designed solely to make money at your expense, Adware will pop random ads up when you least expect it. You'll be visiting Yahoo.com and all the sudden a porn banner will pop-up. Your child will be reading something on National Geographic's site and a bright blinking banner advertising an Internet Casino will appear.
These programs are hidden within Windows, but will do major damage. Granted they rarely hurt your PC permanently, but they will slow it down and make it almost unusable most of the time. The programs are often memory and CPU hogs and are poorly coded, so your PC usually becomes unstable.
Spyware - Most people use "Adware" and "Spyware" synonymously, but I consider Spyware to be a more intelligent version of Adware. Spyware is the nastier of the two, and will collect personal data about your PC and your habits in order to make even more money. Advertisers make more money when you are interested in the ads, so Spyware collects information about your Internet browsing habits in order to sell this information for more dough. For example, if you visit car sites a lot, the random ads will be car ads. This way they'll also seem more fitting so people won't question the ads as much. It's believed that Spyware designers also sell these habits to retailers in order to gather demographics.
Spyware and Adware can both hide themselves many ways. The most common is to load at Windows' startup and stay resident in system memory. Some Spyware will appear on the navigation bar in Internet Explorer and since most people cannot figure out how to remove it, they will be stuck with it even if it's right in front of them. Still, other users won't even notice the navigation bar. This type is generally the worst when it comes to slowing a PC down.
Take Care,
Richard