It sounds like you have somehow removed some core Windows fonts. ie uninstalling an app etc.
Tahoma normal is now being shown as Tahoma Bold, Arial normal is now arial italic etc.
Got to Control Panel\All Control Panel Items\Fonts and see if those fonts are still there. If not:
unzip and copy the attached fonts to the C:\Windows\Fonts directory to see if restores everything back to normal. Fonts.zip