Inside your PC
Not everyone can recite the MAC address of their PC's network card or the driver version for their graphics board (not even me). If your computer is hiccuping, knowing exactly what hardware and system software you have inside your PC is invaluable.
For troubleshooting, being armed with such details is ideal. You can pass them along to tech support, to your computer guru (no, definitely not me), or to an online help forum such as the PC Advisor Helproom (the largest technology forum in Europe).
If you want to visit just one site to test your PC, I'd recommend PC Pitstop's Full Test. This comprehensive tool not only gives you an unbeatable hardware report but also alerts you to various potential problems that could be harming performance.
After I used it recently on my machine, PC Pitstop recommended 11 fixes, gave me a 35-count list of system specs, provided an even longer list of installed hardware, told me what drivers I needed to upgrade, tallied almost 40 performance results, and - get this - compared my PC's ratings with those of all the other computers that PC Pitstop has tested.
If you create a free account, you can save the reports and refer back to them. One warning: if you have a pop-up or ad blocker, disable it before starting the tests.
You can put your PC on the couch and run the free Belarc Advisor to obtain an inventory of your system's hardware. But that alone is no big deal. Where Belarc shines is in its comprehensive analysis of the software installed on your PC, including the version numbers.
Equally useful is the report of Microsoft security hotfixes that are missing from your system, as well as the list of software licences. After you run the scan, save the HTML page in a safe spot so that you can refer to it if the worst happens.
SIW - System Information for Windows
You can ask the SIW tool anything about your PC - it'll give you a list of regional settings or scheduled tasks, or which DLLs are loaded and which are shared. How about which video and audio codecs you have installed, or details about your network or open ports? SIW has everything, offering lists that you can easily access from toolbar shortcuts or from a panel with a hierarchy list.
By the way, because SIW is a stand-alone utility that you don't need to install, it'll run directly from a USB flash drive, a floppy (if you still have one), a burned rescue CD, or a network drive.
Be careful: unless you uncheck the Add Crawler Toolbar to IE and Firefox box during the installation, SIW will automatically install that toolbar and make it your default search provider. Not a cool thing for such a useful tool to do.