Urgent help needed for a friend concerning malware

[#1]

Hi
This is quite an odd question as I immediately thought my friend had a malware virus, but having thought about it I'm not so sure.
The person has a Yahoo account which she uses via a web browser. This week a spam email was sent to each of her contacts without her knowledge using her account.
1) Do you think her account has been compromised - in which case we will delete it and set up a new one
2) Is it possible malware has caused this on her PC - using her browser to send out the emails. What do you suggest I use to scan her PC with? (Malwarebytes?)
I'd appreciate an early response as I'm due to see the person tomorrow. Thanks.

[#2]

I'd suspect number 1. to be most likely though changing the password should be enough to re-secure it.
I'd run scans with a number of virus checkers as a precaution anyway though.

[#3]

Thanks - I thought I was on the right lines of thought

[#4]

I agree, this happened to my Gmail account once. I just changed the password which resolved the issue.

[#5]

There seem to be quite a few cases of hijacked Yahoo e-mail accounts of late. One of the mailing lists I subscribe to has been affected.
As w23 said, changing the password should sort it out.

[#6]

The same with Facebook accounts. I've changed a few passwords lately.

[#7]

Thanks everyone - I'll run an anti malware program and get her to change the Yahoo password (or we look at free alternatives)

[#8]

I missed this thread when it was originally posted but thought I should mention that this problem is very common at present.  I would advise anyone with a Yahoo or BT email account (BT email is administered by Yahoo) to change their password.  Yahoo either have a security hole or they had their password database stolen. 

[#9]

Thanks very much. In the end we closed the account & set up an Outlook one with https & a double lock password.