API Monitor is an extremely powerful tool which can track and display the Windows API calls particular applications are making. It's aimed at developers, but experienced Windows users may also find the program handy for troubleshooting purposes.
To try the program, we unzipped the download file and ran apimonitor-x86.exe as an administrator.
Next, we told the program what category of API calls we'd like to track by checking the "Internet" box in the API Filter window.
We then launched Firefox, at which point API Monitor asked if we'd like to watch it: we clicked the Monitor button.
And that was it: the Summary window immediately listed all the calls Firefox was making, showing us how it was checking the cache, processing various URLs and more.
If you know what you're doing then you can drill down into each call to find out more. Just click on it and check the parameters window to see the addresses, flags, strings and whatever else might have been passed. A Call Stack window provides extra detail, and right-clicking displays even more options. (You can open an MSDN window with help, set a breakpoint, define what's displayed for each call, and so on.)
We just selected Internet API calls as an example, but there are plenty of other categories you might want to watch: graphics, devices, documents and printing, or audio and video, say. You can monitor an entire category, if you like, or zoom in on particular functions - whatever suits your needs.
Of course if you're not a developer then most of this won't mean a great deal, but you may be able to pick up some useful clues. If you think a program is crashing to some drive-related issue, say, you could try monitoring the Data Access and Storage category: while the reports are complex, they'll highlight any errors being returned to the program from Windows, which may help you better understand what's going on.
Please note, as an alpha version of a very low-level-tool there's the possibility that API Monitor may crash an application, or perhaps your entire PC. We had no problems, but play safe anyway: don't use the program if you have any unsaved work in other applications, or while you're running a critical system task.