So all the parts for my new PC have turned up today!

Before I get stuck in building it, I just thought I'd ask, when I install Skyrim on my new PC and it updates from Steam, will it automatically download ALL the DLC I've purchased? If not, how do I do this manually?

Yes, it will download all patches and DLC.

games and dlcs are tied to steam account and not individual PC. just buiild your PC install steam and install skyrim it will download everything

If your mod mix is stable, or all you have are the DLCs, then you can just grab the data files folder on your current rig and copy it over to the new machine. There is even a way to move the Steam installation to a new machine. This will save you a ton of time as you don't have to re-download everything. Just basically the Steam Client and validate files on the new rig. Search at the Steam site on Moving Steam to a New Computer and you will find the pages on how to do that.

Thanks, I have no save games at the moment but was planning on a fresh install for the new one (as I wasn't 100% on how to use mods when Skyrim came out.)

When ever I build a new rig, I mount my old OS drive in an external enclosure, you can get one for like $20. That way I can grab things like my Mozilla profile or email databases or whatever off of the old drive. I never seem to grab everything I need ahead of time, so I just don't even bother anymore. Just grab it off the old drive when I done with the new rig.

Unfortunately my old rig became infected with a virus, as my not so technically minded girlfriend turned off my anti virus and for some reason (even though I used the admin account) I didn't have the "permission" to quarantine or delete the trojan!

Lesson number 1: Do not EVER let your partner use your gaming rig unless supervised!

BUT, I shall bear this in mind for future builds