Check the capacitors on your motherboard and other components. Also make sure there is nothing burning inside (no white smoke).

Caps are all fine I check them regularly, no smoke, just an odor. Not powerful, just enough to smell it.

It might be the smell of new (in which case the smell would be stronger if the new components are under load).
Are your temperatures OK? Voltages?

I don't have any programs that monitor temps. But it makes the smell as soon as I turn it on in the morning, so the new smell sounds like the cause. I just wondered if anyone else here has experienced that since lots here are building computers.

Try to use HWMonitor (http://www.cpuid.com/softwares/hwmonitor.html), Aida64 (http://www.aida64.com/), SpeedFan (http://www.almico.com/speedfan.php) or some other program like that to monitor your cpu/gpu/mbo/hdd temperatures and voltages.
Check temperatures in idle and in load.

As for the smell, my Corsair CX400 PSU produced a smell like burning plastics (but almost unnoticable) when under high load (heavy gaming) for the first few days when it was still new...

what does it smell like?

my cm690 II case had a rubber smell when i got it

Most components that run hot have a smell,especially the new ones, its nothing to be worried about, I have a computer thats over 5 years old and still has that smell, more noticeable in the summer. No thermal sensor will be accurate enough to see if it is a capacitor, transistor or resistor has failed or will be failing and usually by that time if one does fail the system will crash or short out before you catch the smell in most cases and in some rare instances catch fire.

Yea, that "new smell" can last a while, but keep your noise on it ;) If it starts to smell worse, something is wrong, likely suspect is the PSU, but as you know it could be many other things.

Thanks everyone.

I worked in a tech plant for a few years. That smell is toxic fumes from solvent residue that they use to clean the PCB's. If the smell does not go away after a few days or gets worse then you might have problems.

IIRC, my sense picked up that smell when I bought the PSU 2 years back. It dissapeared months later when I put all the components back into the case after I bought it later.

Just set up Seti@home on the GPU(s) and CPUs and smoke it out, leave a window open.
Gitterdone.
Wouldn't hurt to keep it running after the smell goes away either, crunch for science

I LOVE the smell of freshly heated PCB in the morning :) That odor you're smelling definitely sounds like that's what it is. Completely normal, but sometimes people don't smell it in your situation because the pc shop did their job correctly and ran it through some burn-in testing. Sounds like yours just got done and left at that. Just one of many, many reasons I always build my own.