Avast will not install

[ANTI VIRUS]

Trying to install avast on computer with 32 bit vista.  Uninstalled Norton first.  Downloaded and ran the installation software, but the installation hangs up when a Windows window pops up saying "preparing installation".  Nothing happens after that.  I've had to use task manager to close the installation windows.

Any ideas?

[sded]

Do you have any other security software still installed?  Did you use the Norton Uninstall tool at http://service1.symantec.com/Support/tsgeninfo.nsf/docid/2005033108162039 ?  Norton leaves behind a residue that often interferes with the installation and operation of other security software.