I would not use the computer or the account, until I got everything cleared off - you do not want to infect many, many people, which is how the virus spreads.
I would go to the library at your school or University and read up on malware, viruses, and the like. I would check out a book or two on the subjects (your professors will be pleased). Arm yourself with information.
Set up a new computer, email address, and password (strong - with letters and numbers) Set security high, NEVER allow Pop-ups, and only allow Cookies “from sites you have visited” Set History at 3 days. Never open an email from someone you don’t know, or that sounds suspicious in nature - send a new email to that person asking them if they had sent out the email.
Install as much protection as you can. I would not use a wireless router, especially if I were in a crowded environment, as others can actually get on your computer while it is not shut down.
The other answers were great. I just wanted to add a few important things
Good luck

Either this is a virus, or someone gained access to your mailbox.
First - chance your mail password.
Then - run a scan of your system.
Some ‘ads’ ask you to enter your mail password and username for certain offers. The people from these ads use your data to send mail to your contacts.
If you run a scan on these mails, you can see where they were really sent from (via spamcop.net for example).