I might be completely wrong on this ( it wouldn't be the first time ) but I don't think Windows 7 has an all-in-one hardware diagnostic tool. You can access some individual tools like Memory Diagnostic, Check Disk, etc but I think many people prefer using 3rd party tools. One of the Forum experts prepared a troubleshooting tutorial that contains links to some respected and recommended diagnostic tools such as Memtest86+, etc.
Windows 7 does have the ability to run a System Diagnostic Report which is a part of Performance Monitor. It can display the status of hardware resources, system response times, and processes on your computer along with system information and configuration data. The neat trick with the System Diagnostic Report is that you can use it to gather troubleshooting information on-the-fly while the problem is occurring.