Always verify your assumptions. It will save you time and headache. I'll explain now.
Building a PC
I recently built a PC from parts I bought online. It had been 5 years since I've built a PC at the time so I'd forgotten my good habits. I put everything together: RAM, CPU, HDD, GPU, motherboard etc... and it booted up and POST'ed no problems. I thought I was good to go at this point. I managed to install windows and a bunch of drivers, but then I started getting random freeze ups.
I thought perhaps it was one of the drivers or something specific to windows 7 (this is a gaming box, not for work). So I did a little digging and actually ended up reinstalling windows bare to rule out any driver issues. Long story short, while the box would work fine for about 10 minutes, shortly thereafter the box would freeze up.
It was at this point that I decided to get a boot CD with some troubleshooting utilities. I chose Ultimate Boot CD because it's free and has almost everything you could need for diagnostics.
For whatever reason, when I built the PC I did it wrong. I skipped the testing phase. I didn't test first. So now I was in a situation where I was digging through everything to get to the root of the problem (and hopefully there's only one!). First I tested the HDD using HDAT2. Many hours later, it came back clean. Then I tested using Memtest86. Within a couple of minutes it detected an error. So I pulled the 2 RAM sticks, and tested each individually, and determined that I had 1 bad stick of RAM.
The Point is
If I had built my system right: plug in as little as possible to get it to boot and run diagnostics before installing software, I would have found my problem much faster and saved hours of time. My problem here was that I assumed my RAM was good. So even when you're not programming it's important to verify your assumptions and don't forget to test.