Question

I might be totally wrong on this but I don't think you'll find an all-in-one diagnostic tool for Windows 7. There are several tools or utilities that can be accessed from within Windows 7 for specific diagnostic checks. To name a few:

Memory Diagnostics Tool

Performance Monitoring Tool

Resource Monitoring Tool

Disk Check Tool

System File Checker

Event Viewer

I'm sure I've neglected to include some others but a google search for a specific problem would probably give you some more possibilities. For example, if you think your hard drive is going bad, you could google "Windows 7 hard drive diagnostics tool" and you'll probably find SeaTools by Seagate as well as other tools.

Hope this gives you a starting point.