Computer randomly shuts off?

Hi everyone- I have quite a new Acer laptop which runs on Windows Vista, purchased September 2007. I've never had any problems with it, but in the last month it'll shut off randomly in the middle of activities.
I wonder if it's over heating? It's turned off particularly frequently if I'm in the middle of converting music from WMA format to go into iTunes, or if I've been using Channel 4 On Demand to watch episodes I've missed on TV. I'm not sure how to tell if it's over heating or not, the temperature itself doesn't feel particularly hot, just warm (although I thought this was the usual case). I've been using iTunes all year, along with Channel 4 and but it's only started recently.
It will just completely shut off in the middle of activity and will take a couple minutes to even let me turn it back on. Has anyone heard of anything similar? I have full insurance coverage in case it's a problem with the computer, but I wanted to check if the problem could be fixed by maybe buying a base with a fan on top of it, I know people who have those which say it helps. Any help would be appreciated as I'm quite an amateur when it comes to these kinds of problems. Thank you very much!

 

Have you tried speed fan that can detect all sorts of temps try that and check for dust in the heatsink

 

First of all, thank you so much for the quick replies! I've always found this site extremely helpful.
I've downloaded SpeedFan and will see if that solves it.

I'm not sure where to find the heat sink and check it for dust? Sorry! Any advice on that?

About the surface- I never place it on my lap or any soft surface, when it's shut down it's always been on my desk, I think it's a mix of wood and plastic. It's a s strong material and doesn't seem to heat up too easily.

Thanks for the instructions on Windows Defender, I opened it (which said it was switched off actually!), and gave me the following list:

- Microsoft Media Center Tray Applet
- Microsoft Userinit Logon Application
- Microsoft Windows Explorer
- Microsoft Windows host process (Rundll32)
- Microsoft Windows Media
- Microsoft Windows Sidebar
- Windows Defender

- Intel (R) Common User Interface

- HD Audio Control Panel

- Synaptics Pointing Device Driver

- LiveUpdate Notice
- Symantec Security Technologies
- Symantec Shared Component

- Acer Launch Manager

- N/A
(the file name says napster, even though I uninstalled it and removed it a few months ago.. it doesn't give the option to remove, enable to disable it, along with QuickTime, iTunes and a number of others)

- iTunes
- QuickTime

Sorry for the long list, I thought it may be of help. I had used a program a couple months ago off this site to try reduce the number of unnecessary programs running, I wonder if this is the cause for the random shut downs?