A long time ago with Dorothy (HP Pavilion A6000), I noticed my memory count in Dxdiag and Windows System said I had 2GB instead of 4. Then I started getting bluescreens, followed by me panicking, formatting, system-restoring, etc with no avail. My computer finally died and I left it in my closet, lifeless, for 7 months.
After the 7th month I decided "forget it, there's gotta be something I'm missing here" and opened up the PC again. I powered it up, got BIOS and then a blank screen as usual. I thought "What can I do with the BIOS then?" so I ran HP's Diagnostic program, it tested the memory and failed. So it must have been the memory. I took one stick of RAM out and bam, it passed the memtest, so I reloaded and I could reinstall Windows.
This was way before I had any experience with hardware ever, so I was pretty novice about the whole thing.
anyway, more recently? I totally deleted ExplorerFrame.dll while trying to replace it and closed Explorer.exe, and then Explorer wouldn't even open. So I had to whip out CMD and copy a backup into place via command prompt. Luckly it worked...