HTPC not booting after overnight shutdown

Hi all, having a problem with my new HTPC build. Specs are: Asus p5ql pro mobo Core2Due e5300 2.8GHz Samsung 1TB 7200rpm 32MB HDD Sapphire HD4550 Gfx card 2 x 2GB Mushkin DDR2 RAM LiteOn BluRay DVD ROM Silverstone La Scala SST-LC10B-E HTPC case. Be Quiet! Straight Power BQT E6-400W 80plus PSU Vista 64bit OS Assembly and OS installation went fine and is was working no problem whilst I configured Mediacenter, iTunes, other software etc. However, if it is shut down overnight, it will not boot properly in the morning. The HDD whirs, the case and CPU fans turn, but the front panel LED does not light and I get no video output. The only way to get it to boot, is to reset the CMOS, after which it runs fine. It is even OK if I do a quick reboot or evn if left on continuously. The problem only manifests after a lengthy shut down. Could it be a faulty battery, or someone even suggested a new PSU. Thoughts/suggestion welcome

Is your PSU brand new? going by specs, it should be enough for your system, but it's cutting it close on the 12v rail amperage. It's also a split rail which isn't really ideal anymore. The problem you are having seems to be related to the power draw spike when you first power on your system. Why resetting the CMOS allows you to boot up I can't explain, but the first thing I'd try if you have an equivalent or better PSU around is to see if the problem goes away with a different PSU.

The reason I'm focusing on the PSU is that it's a brand I'm not familiar with. Even if it is indeed a reputable brand, there is still the possibility that it's a defective unit.

As for explaining split rails, the people over at PC Power & Cooling do a much better job of it: http://www.pcpower.com/technology/myths/
look at question 8.

I'm not much of an expert when it comes to high quality AND quiet PSUs, lately I've been using units from Corsair, OCZ, PCP&C, Antec. I would check out the list at silentpcreview: http://www.silentpcreview.com/Recommended_PSUs and stick to more recently reviewed units that have a single 12v rail design.

I believe so. If you take a look at the box in store, they should give you a description of the power specs somewhere on it. If it says something like "...+12v 30A..." that's a single 12v rail, or possibly dedicated 12v rail as they put it. If it's something like "...12v1 18A, 12v2 18A..." those are split rails, and not really ideal for newer graphics cards.