my yahoo webmail seems to be sending out spam. Is it my problem or their's?

Its your problem

Follow these instructions

http://help.yahoo.com/l/us/yahoo/security/account_...

Somehow someone or a virus has accessed your account

Same thing happened to me right now. It sent out Emails to all my contacts. I emailed yahoo and supplied the "full header" and the email that was sent out. In my case - I think it may have been to a craigslist post I responded to. Even though it looked like legit yahoo e-mail address. For the time being I deleted all my yahoo contacts and changed my password. Run your virus scan and spy ware software. I also read that yahoo updating it's systems may have something to do with it, but I don't know how accurate that is. If you had personal information such as credit card info it might be a good idea to notify the credit card company just to give them a heads up. It is really nerve-wracking - I hope this kind of helped.

Sounds like your E-mail account has been compromised, or you picked up a virus, Chuck.

Immediately have your computer checked for a 'bug' or 'bugs.' If you have the proper programs, do the scans yourself, if not you need to take your computer to some one who can.

Once it is clean of pests, install a firewall and an anti-virus program if you do not have one yet.

I strongly recommend a program for malware and adware also.

After you finish this, then go in and change your password, using a good strong one.

Also, check your account information and make sure no information was changed, if it was change it back immediately.