Is there a tool like StartupMonitor for Windows 7?

WinPatrol will do this for you. The free version is sufficient, I ran it for years and it did just what you are describing, among other things...

It runs in the background and a dialog box pops up with several options whenever something wants to modify the startup entries..

_{I am not affiliated with this product in any way, shape, or form.}

Another option for analyzing your start up (not so strong in the notifications area but is a powerful tool, nonetheless):

AutoRuns

This utility, which has the most comprehensive knowledge of auto-starting locations of any startup monitor, shows you what programs are configured to run during system bootup or login, and shows you the entries in the order Windows processes them. These programs include ones in your startup folder, Run, RunOnce, and other Registry keys. You can configure Autoruns to show other locations, including Explorer shell extensions, toolbars, browser helper objects, Winlogon notifications, auto-start services, and much more. Autoruns goes way beyond the MSConfig utility bundled with Windows Me and XP.

I use Spybot v1.62, I turn off most of the Spybot features but use Tea Timer, Tea Timer will notify you when any critical registry entry is added or changed which includes any startup items, allowing you to accept or deny the change. Been using it for years on XP and it works just fine for W7. Invaluable tool that I cannot live without.

will stop toolbar installations (among other things) when they try to install silently with other software.

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I set Spybot Tea timer to Paranoid mode for best protection of the registry, this can be done from the system tray icon when Tea Timer is running.