I have an old Dell Dimension 5000 PC and I hadn't used it for a while. I'd had the Windows 8 preview (yes Windows 8 can run on a really old Pentium 4!!!) on it but now I was thinking to install something else. When I switched it on all I saw on the screen was a black screen and a white cursor blinking in the top left. No BIOS messages at all.
I opened it up and looked at the four memory slots. They are in pairs, two black and two cream. I had both filled, two original 512MB cards and two 1GB cards. I had a gut feeling the original 512MB cards were unhappy so I removed them. I switched on and hey presto my old Dell came to life!!!
Now my old PC is running with just 2GB of RAM and not 3GB but it doesn't matter. I'll be installing Crunchbang Linux or similar anyway.