HijackThis!
HijackThis! is an amazing program that lets you take back Internet Explorer if it's ever been hijacked. You can tell a hijacked browser because it will constantly take you places you didn't intend for it to go, and you'll get a tremendous amount of pop-ups on pages that don't usually have them. It's a good idea to check HijackThis! frequently if you use Internet Explorer.
Install the Program
You can download a copy of HijackThis! here. Once you have the file downloaded, you need to create a directory for it. You can do this by right clicking the Start bar and choosing Explore from the menu, like this. Then navigate to Program Files using the left pane, and right click in the right pane and select "New Folder", like this, and name the folder HijackThis. Copy the HijackThis file to your new folder and then right click on it once it's there, like this, and choose Send to Desktop (Create Shortcut). You now have on your a desktop, an icon that will let you run the HijackThis program.
Running the Program
Double click the icon and you'll see this. Click the big bold Scan button and you'll see something like this. The key to this program is knowing which lines to select as bad and which ones to leave alone. Look at my screenshot, these are all good lines to have. The key to being able to tell is whether it looks familiar. Anything that comes from a company you know, like Google, SpyBot, Microsoft, Apple, Macromedia (Flash Player), etc. Anything that looks suspicious, such as only numbers instead of websites, or lots of weird characters may just be a problem. If in doubt, fill out this form and I'll try and help you out. Otherwise, just select the lines that you want gone, and click on "FIX Checked" and that's it.