bcebulla on "HTTP Error 500 upon second install"

I had multisite activated on my wordpress site. Decided I wanted to go back to just a single site.

So I deleted all my wordpress files via FTP. Then I re-uploaded default wordpress 3.5.1 via FTP (edited wp-config.php accordingly).

Now I'm getting the "HTTP Error 500" when trying to install at /wp-admin/install.php

I tried this php memory trick: http://wordpress.org/support/topic/500-internal-server-error-at-wp-admin-after-install?replies=9

But that was to no avail. I've searched other threads on this topic, but haven't come across anything that seems similar -- particularly because installing WP first time on site ~6 months ago went just fine.

Any help is appreciated.