Read the second book in the Ciaphas Cain series. They discover a Necron tomb. I've never read any books about guard fighting dark eldar. Maybe it's just too rare.
The Imperium doesn't distinguish between Eldar and Dark Eldar a lot of the time, because they simply don't know the difference. The Dark Eldar fit much more with the propaganda regarding the Eldar, really, so that makes it even harder for a Guardsman to say, "Hey, these guys don't have all the shiny, gem-encrusted stuff..."
Incidentally, the Guard have, in novels, fought the Dark Eldar. The first of the Ultramarines novels, Nightbringer, has a situation where a Guard column gets involved with the dark 'uns. There are a couple other instances I remember, but I can't remember which specific book they were in.
The Guard, I think, are much more likely to face Dark Eldar/Eldar Pirates than Craftworld Eldar armies, in the end. The Craftworlds don't send out an army unless they absolutely have to, and usually, that involves defending a Maiden World. If the Imperium figures out that the Craftworld Eldar are up to something and are attacking a planet they've laid claim to, the Space Marines usually respond, since the Craftworlds are significantly more dangerous fluff-wise than bands of pointy-eared pirates and slavers. The immediate response forces for slave raids from the Dark Eldar are going to be the Planetary Defence Forces, which are a seperate entity from the Imperial Guard to a greater or lesser extent, although on the table they're identical.
Both the countless PDFs and the Imperial Guard are supplied by the Departmento Munitorium identically for the most part, and both fall under the jurisdiction of the Warmaster (not to be confused with Horus, though it's the same title and position) on Terra. The difference is that PDF forces are permanent garrisons drawn to protect a single world, which the regiment is both raised upon and never leaves. The Imperial Guard regiments are mobile and are redeployed where needed. For example, you can have a Cadian regiment which is a permanent garrison on a world in the Cadian system, such as Kasr Sonnen, and they'd still be described as the Cadian whatever-ith, but administratively, they're PDF forces. The Cadian 8th, on the other hand, which is the famous regiment that appears as the "standard" Imperial Guard army in pictures, is an Imperial Guard regiment and detachments from it are sent to the appropriate warzones where needed.