I thought this was a quirk the first time it happened but now it's occurred twice in the past week.
Setting up two new, out of the box computers with the normal stuff: avast!, Ewido, Ad-Aware, Sbybot S&D, Spyware Blaster, etc. When it came time to install WinPatrol the installation processed stopped midway with no explanation. This happened with multiple attempts, after reboots, on both machines.
The problem was resolved by turning off all avast! providers (turning off just the Standard Shield did not work). Both computers have the default avast! installation with no blocked operations checked in the behavior blocker and "Allow the operation ..." checked if avast! can't ask what to do. Everything except WinPatrol was installed without any problems.
Just thought I would post this in case anyone else runs into a similar problem.