Why Does My Computer Go Very Noisy When It Is On The Internet?

It sounds like your computer is suddenly doing lots more work, so the best thing is to check what is running at the time.

Bring up the task manager by right-clicking on an empty part of your taskbar (the strip along the bottom of the screen), left-click on Task Manager.  In the window that pops up, click on the Processes tab.  You should see a table of processes.  Click on the CPU header a couple of times, so that the top process in the table is the one with highest CPU.  The System Idle Process is nothing to worry about, but make a note of any other process which is consistently at the top.

Post back here what you find and I'm sure we can solve your problem!

Update, based on your comment: Since it's firefox causing the high CPU usage, I would check first what toolbars and add-ons you have installed.  Check for add-ons in Tools:Add-ons, and for toolbars in View:Toolbars.

I personally have had a lot of good experience will Dells, but it must be annoying for you having an unreliable build.  I'm sure that's nothing to do with this problem though.

Does this problem only occur on certain sites or all the time?

Also if you go back into the task manager as described by MadBiker, you can see the Mem Usage for firefox or IE, and at the bottom of the task manager is the Commit Charge. If you can post back those figures hopefully we shall be able to find the problem this time.

Also I would suggest removing the Yahoo! Toolbar as that used to cause a few issues for me in the past.

UPDATE

There is no problem with your computer, it is doing what it should do. The noise you are hearing is the internal fans keeping everything cool, the computer has to work harder on sites like cbeebies because of all of the graphics the site uses.

Also if it is a hot day or the room the pc is in is hot, the fans will have to work harder.

From the figures you have provided, your pc does not appear to be struggling, and seems to be coping well.