>> so I'm still a little bit mystified as to which is the official default system font. <<

Tahoma is the font used by Windows 2000 and XP. However, to get this font you need to use the typeface "MS Shell Dlg 2". If you use the "MS Shell Dlg", the old font is used for backwards compatibility. "MS Shell Dlg 2" is only available on 2000 and XP, so if you want to target older platforms you need to provide appropriate code:
Code:
	const INT     ITEM_POINT_SIZE = 9;
	LPTSTR        szFont = TEXT("MS Shell Dlg");
	HFONT         hFont  = NULL;
	OSVERSIONINFO osv    = { sizeof(OSVERSIONINFO) };
	HDC           hdc    = GetDC(hwnd);
	INT           nFontHeight = MulDiv(ITEM_POINT_SIZE, GetDeviceCaps(hdc, LOGPIXELSY), 72);

	GetVersionEx(&osv);

	if (osv.dwPlatformId == VER_PLATFORM_WIN32_NT &&
	    osv.dwMajorVersion >= 5)
	{
		/* On Windows 2000 or greater. Use the new-look font. */
		szFont = TEXT("MS Shell Dlg 2");
	}

	hFont = CreateFont(nFontHeight, 0, 0, 0, FW_NORMAL, FALSE, FALSE, FALSE, DEFAULT_CHARSET, OUT_DEFAULT_PRECIS,
	                   CLIP_DEFAULT_PRECIS, DEFAULT_QUALITY, DEFAULT_PITCH | FF_DONTCARE, szFont);

	ReleaseDC(hwnd, hdc);
In XP, on most menus, messageboxes and button labels, Tahoma is used, however sometimes in editboxes and text boxes, MS Sans Serif is used.
Well, this may be deliberate (a decision has been made that Sans Serif is more appropriate), it may be a mistake (Microsoft programmers are not perfect) or it may just be that the code has not been updated from older platforms.