Media Player adding black bars + other probs

Recently media player stopped playing avi's (other movies ok). After a day of tinkering I decided to do a fresh Windows 7 install using my fairly new 2nd HDD (previously used to test Windows 8). I used the USB install method from this forum. Bizarrely media player didn't work at all on the new install (media player has stopped working message). With repeat installs, everything else seems ok except media player. The best I've managed is to get media player working, but adding black letterbox bars top and bottom (see attached jpg). Updates usually make this half working wmp stop all together.

Things I have tried -

disable overclock
different USB pen drives
different iso (from links in this forum)
using onboard graphics instead of Radeon HD card
with/without gfx drivers
with/without MS updates
brand new HDD
sfc /scannow

also -

media center plays tv ok, as does media player classic

the system, a pre-overclocked bundle from Aria in UK, has a huge cooler and good psu and has been running @ 100% (rendering C4D) since new. I've has a couple of BSODs in total that I put down to overclocking (ok on restart), but I'm wondering if I have a motherboard fault - although I can't think why this would only affect media player.

One more thing - when I first built this box I had trouble installing Windows 7 BSOD etc. I turned the overclock off and install went ok. Turn o/c back on and worked fine. Maybe I've had a mobo fault all along?

I was thinking of a BIOS update, but although I can save the o/c data it might not work on newer BIOS (o/c was done by mobo supplier)

Jed, up all night, tearing out hair

Just started memtest - will try your suggestion when done.

indianacarnie - MS Fixit didn't fix it. also 3 passes of memtest = no errors.

Anyone have an idea why a fresh install of Windows 7 should have a broken WMP adding a letterbox to movies ? It might seem like I'm moaning about some black bars, but I don't want to load all my software on a system that might be corrupt.

The only thing I haven't substituted is the cpu/mobo, but I can't see how a hardware problem could repeatedly add 60 pixels top and bottom to a movie clip. Then Windows updates makes WMP stop altogether. Re-install, same symptoms.

I've done dozens of builds, repairs etc but this has me stumped - I'm kinda hoping someone will say 'did you tick the xyz box when you installed'

After about 8 installs, wmp worked fine. I did all the updates and now mp4 plays ok but avi gives 'media player has stopped working'. Thumbnails show which usually means it will play. I guess I'll call it a draw, and file under 'intermittent hardware fault'.

Its like Sherlock Holmes said 'when you have eliminated the impossible, whatever remains, however improbable, must be the truth'