cant install avast

everytime I try to install avast, it installs up till the end (so the progress bar gets to the end of the install) but then just stays there forever.

its stuck on vcredist_x86_sp1.exe

I close the install, it says I can finish it later. So I follow this link- http://forum.avast.com/index.php?topic=58183.0 -click run and it to try and make it finish. It downloads and then i verify but after that nothing happens. What should I do? Why am I having so much trouble?

do you have any antivirus / security software on the computer ?

just spysweeper