A folder has appeared in My Computer under Applications titled "Yahoo Messenger". Inside the folder is another folder titled as a Yahoo Messenger ID that is not mine, but is similar to a user name I often use for other things. Inside here has been saved photos, some pornographic. I have been told that different viruses can cause this to happen. Is this true? If so, how, and how can I stop it?

Well if it is a virus - just make sure your PC is protected well. That means a virus scanner, some spyware removal tools and a firewall would be a smart idea too. Links to some suitable programs are below in my signature (ZoneAlarm is the firewall).

Make sure you have a virus scanner updated and running. Scan your PC for malware using Ad-Aware, Spybot etc. Then hopefully these will pick up what did this to your PC.

Move the whole folder elsewhere and see if it gets recreated.

Hi,
Have you run a virus scan, using your antivirus program, in Safe Mode?
If not, then I would do that first.
Then, I would run a Spyware scan, since you mentioned Porn in the folder.
Here are some free programs, if you don't already have them, and the suggested way to run them:

If you think you already have Spyware/Advertising Ware in your computer, run these as follows:

AdAware at:
http://www.lavasoftusa.com
Download: AdAware_SE V 1.06

CWShredder at:
http://www.trendmicro.com/cwshredder/
Click on the "Remove CoolWeb Search" button to download

Both of the above programs run better and much faster when run in SafeMode.

To get into SafeMode:
Re-boot the computer, and immediately after starting up, Press and hold down, F8, at top of keypad.
When the options show on the screen, use the up and down arrow keys on the keyboard to select
"Safe Mode".
Press Enter

It's best to run the AdAware scan first; 3 times; then re-boot.
Then, run the AdAware scan again 3 times. Then, run CWShredder.
Re- Boot.
Reason for running so many times:
Some of these trojans' files can be deleted the first time; leaving some others; but on re-boot, they re-write the files that were deleted.
Running multiple times deletes most of it the first
Time.

If you wish to have a great program, after you clean out Spyware/Advertising Ware:
This program stops this stuff from getting into the computer in the first place, by placing URL's in the browser, stopping them instantly. One of the Very Best free programs anyone can download!

SpyWare Blaster 3.4

http://www.javacoolsoftware.com/sbdownload.html

Free online scan at:
http://www.trendmicro.com/spyware-scan/

This might take a little troubleshooting to determine why the folder is there.
The above scans would probably eliminate Spyware as being the reason, if it finds nothing.
Best of luck.