Yahoo Companion – YTSINGLEINSTANCE.DLL

Ytsingleinstance.dll and ycomp.dll install with Yahoo Companion, a free, personalizable toolbar that allows you to search from anywhere online, access your favorite Yahoo services, and save bookmarks online that can be accessed from any computer. It also notifies you when new mail has arrived in your Yahoo mailbox. If you want these services, then you'll need to leave this file in place. If not, you can remove it via WinPatrol or in Control Panel > Add/Remove Programs > Yahoo Companion.
http://companion.yahoo.com/. A tutorial can be found at http://toolbar.yahoo.com/ and at http://companion.yahoo.com/config/slv4_page?.p=intro&.src=o.

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