Whenever I restart my Vista computer after installing Apache 2.2, I see a error dialog with no message or icon. Repairing the installation didn't fix the problem.
After the usual ten minutes of head-scratching and searching, it turns out that the Apache installer creates a task in the Windows Startup folder called Apache Monitor, which refers to ApacheMonitor.exe. This is Windows taskbar applet which, I guess, allows you to monitor the status Apache services. Since I only run Apache on my computer for software development, I don't particularly care about running this applet and I removed it from the Startup folder. When I restarted my computer again, the blank error dialog is no longer displayed.
Investigating further, I found that this program doesn't work in Vista.