When I am in 2nd life my computer over heats and shuts down. I have an Alienware about 4 years old.

[Rolig Loon]

Be thankful that it does shut down.  Otherwise you might have to replace an expensive graphics card, or more.  SL will make almost any computer heat up.  It pus a heavy load on a graphics card, which then draws a lot of current from the power supply and converts most of it to heat. 

You can do a few hardware things.  Keep a regular schedule for cleaning the computer.  Open it up and use a can of air to get rid of dust bunnies and cat hair.  Fans are cheap insurance, so be sure that you have several of them and that they are spinning freely.  I have three in my tower (including one on the power supply), plus one that's on the graphics card itself.  Get the tower up off the floor, where it's most likely to get dusty.  Consider getting yourself a new graphics card that is better suited for SL.

You can also do some things in world to reduce the load.  Make your draw distance short (64m maybe) so that your graphics card isn't trying to render quite as much.   Lower the Quality slider in Preferences >>> Graphics too, and decrease the maximum number of non-imposter avatars from 12 (the default) to 5 or 6.

[Rolig Loon]

Be thankful that it does shut down.  Otherwise you might have to replace an expensive graphics card, or more.  SL will make almost any computer heat up.  It pus a heavy load on a graphics card, which then draws a lot of current from the power supply and converts most of it to heat. 

You can do a few hardware things.  Keep a regular schedule for cleaning the computer.  Open it up and use a can of air to get rid of dust bunnies and cat hair.  Fans are cheap insurance, so be sure that you have several of them and that they are spinning freely.  I have three in my tower (including one on the power supply), plus one that's on the graphics card itself.  Get the tower up off the floor, where it's most likely to get dusty.  Consider getting yourself a new graphics card that is better suited for SL.

You can also do some things in world to reduce the load.  Make your draw distance short (64m maybe) so that your graphics card isn't trying to render quite as much.   Lower the Quality slider in Preferences >>> Graphics too, and decrease the maximum number of non-imposter avatars from 12 (the default) to 5 or 6.

[Lindal Kidd]

Alienware computers are generally intended for intense gaming...a desktop model should not overheat when running Second Life.  Open it up as Rolig suggests and check for dust.  Make sure all the fans are working, and replace any that are not.

If all that looks good, it's possible that the heat sink attached to your CPU needs to be re-seated.  Carefully remove the cooler and scrape off any thermal paste on the cooler and the CPU with the edge of a credit card.  Then put a pea-size blob of a good thermal paste like Arctic Silver on the CPU and re-install the heat sink and cooler.  If you aren't comfortable with doing that, take it to a computer repair shop and ask them to do it.

If you are using an Alienware LAPTOP...well, laptops get hot when gaming.  You can open it up and check for dust, but opening a laptop requires an (inexpensive) set of special screwdrivers and plastic pry tools.  If you don't feel comfortable with that, take it to a repair person and have it checked out.  You can get laptop coolers...pads that you sit the computer on that draw off excess heat.

[Claireschen Hesten]

In addition to the great answers already posted if your alienware is a laptop you should invest in a cooling pad often they are nothing more than a plastic surface with feet with a large fan in or three small fans that connects to your usb these fans keep your laptop cool. sometimes they might just be a riser that gives space for air flow under your laptop or a chill mat that that absorbs the heat