Free Avast antivirus, downloaded, installed and disappeared.

Hello,
I'm using Windows XP with Firefox. I have Kaspersky antivirus, but my computer recently slowed down so much I figured I need to try something else. So I downloaded the free Avast anti-virus yesterday, which showed up in my download box as setup_av_free.exe 42.6MB-cnet.com. So I clicked on that and it gave me the option to Run it, so I clicked on that, and it went through an installation process. When that was complete, it disappeared. There was no icon on my desktop or toolbar or Start menu. So I looked for it in All Programs and in My Computer - Add or Remove programs, and still didn't find it. So then I did a search for files with Avast in them, and I found an empty file named Avast5 in Documents and Settings. There's another file named Avast5 in Program FilesAlwil Software that has 57.8 MB, but when I opened it, I didn't find how to open the program. There are also 3 different Avast.setup files. So my question is, how do I open and use the Avast antivirus I downloaded?