Sony Vaio keeps shutting down

Hello, I have a Sony Vaio VGN-CS110E. Just recently I have been having problems with the computer just shutting down on me. When I try to reboot, it continues to shut down. I have to wait a while (like 30 minutes to an hour) before it will boot up again and its hit or miss whether or not it will stay on for a period of time.

I purchased the computer in January of this year ('09). It has worked flawlessly until recently. The computer came with Windows Vista and Internet Explorer as the web browser. Only recently problems started (first) when Internet Explorer would stop working if I was trying to access my school e-mail from home. I could log on, open e-mail, but could not compose or reply. At first it worked, but recently has not.

So, I ran system checks, scans, updated my virus software (with Avira Antivirus....the free download) and assumed everything was up to date at that point. Still Internet Explorer would not work properly. Now, I could do everything else.......surf the web with no problems. The message that would pop up was "Internet Explorer was closed," click for solutions........then Data Execution Prevention would pop up. I don't know much about that, but when I tried to change some things in there.....still no luck.

So, I just installed another search engine.......Firefox. Then everything worked fine for about 2 weeks. I could open, return my university e-mail, etc.

Just recently, my wife's Dell computer went out and she has been using my Vaio. Don't know what she may have added, altered, etc, but the last few times I've tried to use it it's been shutting down. The first time it ever happend (and we couldn't get it to come on) we waited......then got it to come on. My wife immediately did a system restore.......which restored things that I had been out of date, and also made my Avira Malware out of date again. So, I just simply thought if I re-installed Avira everything would be fine. Now there is a message that says: Avira "There are compatibility issues with this program." It pops up just as the computer is turning on after I sign on. I tried to un-install it but for some reason it won't. I also re-installed Firefox (after the system restore).

The ending result lately has been that everything is shutting down. Sometimes if I am trying to watch a video while on-line and I run it through Windows Media Player...in the middle it just shuts the computer down. From that point on its hit or miss when I can get it to turn on again. I don't know if programs are conflicting with each other or what. Whenever updates it says the computer is updated. When I have run system scans it says no viruses found. The last time I tried running a system check in the middle it just shut down again. So, again I'm just confused.

One other note: The fan seems to run pretty consistently. Like "noticeably" more than usual I guess. I first noticed this around a few months ago, but it never had been shutting down like it has been the last 2 weeks.

Can you help?

Thanks.