Vista Black Screen - Random black screen when doing anything

I have purchased an hp pavillion with vista preinstalled I have only had the machine a couple of months and basically every now and again the screen will just go black the machine is still running but the screen just stays black.

If i am playing a music track or something the screen goes black first then the music will stop but the hard drive is still running. If i leave it, it just stays like this - it's not hibernating or on standby because I could actually be using the machine at the time when it the screen goes black. I have tried pressing numerous buttons, moving the mouse etc. to try and start it back up again bu usually pressing ctl alt dlt twice turns it off, or I just hit the off button until it turns off. However sometimes when it reboots it takes me back to the email i was working on, or the music i was listening to - sometimes it doesn't it just restarts the whole computer.

I am very confused? Can anybody offer any advice?