Motherboard detects RAM sticks in empty slots

Jul 28, 2018
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Motherboard: MSI MS-6590 (Socket 462)

All out of the sudden my old XP build started being very unstable and kept crashing, so I thought it's a RAM issue. However, when I was testing with different stick/slot configurations I noticed that my board was detecting 3 sticks inserted, even though only one was actually in. Upon running Memtest it was just spamming errors in every possible RAM location. Any ideas?

My original RAM config was 2x512 MB (1x single-side DDR400 + 1x double-side DDR400) and 1x256 MB (1x double-side DDR400).