Jm4n
I'm not sure if I'm reading you correctly, but are you accusing Yahoo.com of infecting your PC with Nimda? Nimda affects Windows machines; Yahoo runs FreeBSD on their webservers.
OTOH, if you are talking about Yahoo Mail, that's your responsibility (and that of whomever sent you the virus). Not to mention Microsoft, since your browser allowed a virus to infect you.
In any case, if you read the EULA for Windows, IE, and so on, you will find that they have covered themselves in that department.
Give us some more details here, I'm extremely curious as to how you came to the conclusion that Yahoo has infected your machine with a virus...