Since I installed my Raptor (74GB) last week, first thing that happened was that it wouldn't boot up and then it would just crash on me. So I changed the SATA cable and it fixed it.
However, now it keeps "powering down". It makes a noise as if the computer is shutting down but it doesn't and everything freezes for maybe 5 seconds and comes back to normal. I never hear the HDD spin up again, but maybe 15 minutes later it'll "power down".
Other than that everything's fine with it, just wasn't sure what might be causing this. shrug
Try this (this is is xp so maybe a bit different for vista)
Got to Display Properties, and then Click on the Screensaver tab.
Then, click on Power... button in the Monitor Power section.
There, in the Powere schemes tab, there is an option for "Turn off hard disks" where you can specify whether or not it turns off after a certain amount of time so check there.
Is it your boot drive? If it is your boot drive it should never power done and there could be a problem with it. But Raptors have a 5 year warranty so it is not a big deal to RMA it.