If folder options are not set or changed to allow for hidden folders to be seen you may have a problem finding certain things. Startup folder is a "special system" folder and would fall into the "system hidden" category.
Are you also missing the start button>all programs>startup folder shortcut? http://www.checkcomputersetup.com/fi...tup_folder.JPG
If it's not missing, right clicking on the Start button, left clicking Explore should take you to C:\Users\user name\AppData\Roaming\Microsoft\Windows\Start Menu. The Startup folder should be there.
Also the shell:
command can be used to open a special folder directly from the Start button>Search menu or from the Run dialog. For example, typing the command shell:startup
will open the Startup folder (%userprofile%\startup) of your user profile. If Yahoo Messenger startup is the focus, I believe this is an option found within Yahoo Messenger its self. Somewhere, with in options, will be a checkbox to enable or disable Yahoo Messenger from starting up when windows is started.
(look for "general" options within yahoo options.)
I just noticed, while in the virus/malware, you have posted and suspect a virus or malware infection. Yahoo may be involved. The wisest thing to do, at this point, is continue to remove or eliminate viri as a possibility. Anything else you do, may spread infection, complicate the cleanup or prove to be fruitless. Hope for the best and good luck to you.