I just installed DX 9.0b. I had the computer running for about a week till then and restarted because of the installation. I took a few looks inside dxdiag and restarted again. But the System didn't boot up, instead CHKDSK came up and said it needs to repair C:
Well no problem, it repaired something called "a broken attribute entry" (<-- translated, don't know the exact english term).
Computer restarted again and no problem from then on but what I ask myself is: Why do things like that happen? I still know from the FAT32 time that files can get corrupted after crashes and stuff but why after a normal reboot? Someone can explain that to me?
Thanks!

A broken attribute entry is when the attributes for a file (read only, archive, system file ect...) are not found.
Generally not bad as it will be given new attributes and sent on it's way.
There are no hard and fast rules about why this happens, the mechanics of a hard disc are very sensitive and how they store so much data in such a small place is amazing, but unfortunatally anything from a slight knock to cosmic ray from space can affect the data on the disc.
The only thing you can do is to keep backups of your data and programs so if something really bad happens it won't be too painful to get your pc up to speed again.

Your OS, all installed softwares, and all data are stored on a hard drive that is composed of many thin disks covered both-sides with a magnetic film. The magnetic film is made of minute magnetic particles (microscopic iron rods say) that microscopically can move - - say point North, or Point East.

Minature magnetic heads immediately adjacent to these coated disks embed a magnetic field into the little iron-rods to make them point in the direction wanted. Pointing North is a ONE, pointing East is a Zero, say.

But making a iron-rod point one way, and then keeping it so-pointed over night is not fool-proof. As indicated above, a burst of a stray magnetic field (from Speakers, Fans, Xeon lights) may disturb the rod alignment. Or it may just be a weak iron-rod that will not hold its alignment. The disk coating may be flawed.

A hard drive has tons of errors on it each and every day. There is no way to keep all those billions of billions of little iron-rods to stay put. Some are always going to be pointing where they are not suppose to be pointing. So, the hard drive incorporates error corrections, placed every so often on the disk, which are used to compensate for these pointing errors. But, only so many error corrections can be handled in each sector. You may have had an over-ride.

Or alternately, the OS may have had a hiccup while you wee running, though you did not notice it. The OS may have never really finished the job of properly laying down the file onto the HD magnetic disk medium

Thanks guys, that covers everything I wanted to know!