For the last four years, I've been using a chipped XBOX as my media centre. Running XBMC, it has managed to play everything I've thrown at it. Recently, though, it has had a real problem playing standard DVDs. Aside from percussive maintenance, the only way that I know to fix something is to crack it open and have a fiddle!
I've opened my Xboxes several times before - if you're new to it, you'll need a TORX screwdriver set (Size 10 & 7 are the only two bits you actually need) and a regular cross-head screwdriver. And nerves of steel, obviously. There are lots of guides to taking apart the Xbox - there's a particularly good one on repairing DVD drives.
As it turns out, my DVD drive was suffering from a 4 year accumulation of dust. Eurgh! Clicking each of the pictures will show you the horror in 5 MegaPixel detail.
Luckily, dust is one of the easiest problems to solve. You just put your lips together and blow...
Actually, a can of compressed air and a lint free cloth is generally better!
A quick reassembly and the DVD drive is as good as new.