We've become used to the idea that the modern PC can give us reasonably detailed information about what might be wrong when things go afoul. That hasn't made end-user detective work obsolete -- if anything, it's made it all the more valuable, since the user now has to diagnose what on the face of it might seem like something wholly undiagnosable. It's not -- it just requires a bit more tenacity and patience than normal.
Remove Or Disable Everything That's Physically Unnecessary
This is a textbook troubleshooting technique, but many people are loath to go to the lengths they need to make it thoroughly effective. A mouse, keyboard, display, and maybe a network connection are all you need to get things going -- and sometimes you can do without the network connection as well. If you have an extra display, unplug it, too -- a second display can be problematic for reasons I'll go into later.
It might able to be useful to go into the system BIOS and disable devices that are not in use if you have the option to do so. Examples: onboard audio or networking, unused bus controllers (e.g., FireWire), or devices that are enabled but never actually used.
Clean Up And Look Around
Now would be as good a time as any to break out the Q-tips and vacuum cleaner. Open up the PC and look around -- sometimes the problem may be something grossly physical that wouldn't come to your attention when the lid is on. Loose or severely bent cabling (especially for hard drives), dust clogs on fans or heat-exchange apparatus, and bulging capacitors should all be considered signs of trouble. This is another reason to run with as little hardware as possible: the less you have inside the system as well, the easier it is to spot problems like this.
Turn Off Unneeded Kernel-level Objects
Aside from hardware, disable any non-Microsoft drivers or components that aren't absolutely essential. One powerful tool that can be used to this end is Sysinternals's AutoRuns, a program that's something of a big brother to Microsoft's own MSCONFIG. AutoRuns covers a great deal more territory than MSCONFIG, and like that program, anything disabled through it can be re-enabled later on without a great deal of hassle; its effects are totally reversible and nondestructive.
