Most spam is sent with fake sender's addresses (this is trivially easy to do). One possibility is that someone you know, or have had e-mail contact with, has had their computer infected with spyware turning their computer into a member of a spam botnet. Botnet is a jargon term for a collection of software robots, or bots, which run autonomously. Specially crafted software compromises the users contacts list and uses the infected computer to send spam all over the world using random addresses from the contacts list (including yours) in the from field of the spam messages.
Another possibility is a spammer is disguising the true sender of the e-mail with your address. This process called e-mail spoofing which is a form of the joe-job, a spam run forged to appear as though it came from an innocent party. In e-mail spoofing, the sender manually constructs the e-mail header and chooses which information (your e-mail address as the sender, for example) to include.
Just ignore the messages unless you have some other reason to think your computer is infected.