Sometime last weekend my desktop font changed for no reason. I can change it back by going into the Appearance settings and reselecting the Windows Aero color scheme. Then next time I log on again, it's changed back to how it was. See attachment for how it looks on screen. The before image is how it is when I log on, after is how it is after I have reselected. If I look at the advanced properties either before or after, it claims to be using Segoe UI, but these are clearly two different fonts.
I have had other desktop settings that do not persist after logging off, like the control panel appearance. When I log on it is on icons, so I change it to list. Then next time I look, it's back to icons again. It's driving me mad.