Startup Defender is a small program that sits in your Windows tray and constantly monitors the startup locations on your PC.
If a program tries to write itself into any startup location Startup Defender will popup a windows and ask if the program is aloud to place itself to start automatically with Windows.
Also you can disable/enable any programs that are currently installed to startup automatically. If there is an entry you are not sure what it is then you can Google it to see if it is needed or even harmful and choose if you want it to load at startup or not.
30 day free trial for Windows XP/Vista, requires Microsoft .NET Framework 2.