It's a fairly common problem. It's likely just a poorly shielded VGA cable and one of the signals is close to a multiple of the frequency you were tuned to.

I was recently playing with a FPGA board and found that it emits quite a lot of RF leakage, stomping over my reception of the 128.000 MHz AM aircraft band which happens to be 4x the 32 MHz master clock on the FPGA. I loaded a Commodore SID implementation into the FPGA and could then hear the SID tunes loud and clear on the scanner. Last year I was messing around with a small Tesla coil I had built, and we discovered it was stomping all over one of the local AM talk stations. My friend tuned it in his truck and could hear the Tesla coil down the street. RF is nifty stuff but keeping it just where you want it can be a real pain.

To clarify, there *are* people who still listen to AM, but they are certainly in the minority. Dozens of unfiltered computers buzzing away in the neighborhood could cause some problems, but running one vintage machine occasionally for a couple weeks while you wait for new caps to show up is unlikely to cause any harm. If it's interfering with something you're trying to listen to, just shut it off.