What is it?
Yahoo Instant Messenger - ypager.exe

What does it do?
ypager.exe - This is the tray bar for Yahoo's Instant Messenger application. I'm a big fan of trillian to connect to AIM, Yahoo, MSN all at the same time. If you have multiple tray bar applications for IM apps you should look into it.

Virus Precautions:
You'll want to keep an eye on this google search for any known viruses. The normal location of ypager.exe is C:Program FilesYahoo!Messengerypager.exe