You need to ask yourselves why. Your account becomes compromised because you have used a password that's totally insecure.
What do I mean by that? You are using a name or a word that is in the dictionary! Makes things dead easy to crack.
What should you do? Get your password changed NOW, before it gets hacked. Your password should appear to be a random set of numbers AND letters (both upper and lower case). Don't just use a word and replace letters with numbers (people often use the number 4 to represent the letter A) because that's as bad.
The people trying to get into your account have already got your email address from someone else who has had their account hacked so they're now after yours. If you know anyone who's had their account hacked then you need to change your password now.
It would seem that they've had all the rich pickings from hotmail and are now starting to target Yahoo more and more.
Basically, once they've got your email address, they have a computer constantly filling in different passwords in the hope they'll get yours. The first things they try are words from the dictionaries (all languages), names, mis-spellings, letter replacements and so on. This is why it should appear to be a random set of numbers and letters.
Please don't try the argument that you can't remember something like that because you don't have to remember it. Another massive don't is let you browser remember it for you. When you enter passwords, you get a prompt asking to remember your password - don't do it because they can easily get them from there.
Your password should look something like this - F5to0%2fmYgZW - the longer the better. I would recommend at least 12 characters.
Another nono with your passwords is use the same one for everything you log in to on the internet because as soon as they've got it, they're into everything you have. Use a different password for each website - email, facebook, twitter .......
Whilst it appears handy to link all your social networking together so you only make one post and it propagates across all your accounts, do you really want someone to have access to everything you own without your permission and make posts on your behalf!
Get a book to record your passwords and keep it at the side of your computer. Don't be tempted to create a document on the pc and keep them there. That's as bad as asking your browser to save them.
Not only will they start spamming from your account, they will delete all your contacts once they have taken them. They will change the settings in your email account and forward all emails to their account, so get into your email account now and familiarise yourself with all the settings, where you can change your password, where the mail forwarding and all the other controls are. That way you can keep ahead of the cyber criminals.
Just because your account hasn't been hacked yet, it's just a matter of time for 80% of you so act now and protect yourself!