As with dc66, Windows uses a specific font for most of it activities. It should change only if the required font is not present, or some utility is overriding that option, like an appearance utility. I will acknowledge, that on occasion strange things do happen, like the anti-virus which hid all the library folders.
Right click the desktop and select personalization, then Window Color, then Advanced appearance settings. Select something in the window that has a font associated with it. Does it show Segoe UI? If not, what does it show, and do they all use the same font.
Browsers have their own font settings but do not normally effect the basic Windows font selections. Language packs may be involved if you are using one of those.
If it is changing 2 times a week, what changes it back?