I wish I could pinpoint which file it was so I could remove it.
If you are seeing this on ALL the default aircraft, more than likely it is either FX_2.bmp located in the EFFECTS\texture folder or HALO.bmp located in the MAIN FSX texture folder.
FSX does funky things with aircraft lights on the default planes. During the day it uses FX_2.bmp and at night many of them switch to halo.bmp.
Not all of the lights switch, but many of the white marker and beacon lights do, and some strobes
Is this seen both day and night?
It is also possible someones addon installed FX files overwriting the default originals which call or use their own textures. Rare, but I have seen that.
halo.bmp is also the airport lights too so if you are not seeing the problem with airport lights and just the aircraft, more than likely its FX_2.bmp