Someone has hacked into my email address book and is sending spam to addresses from my book. How can I stop this?

How can I stop this?

Probably can't, suggest doing the following anyway:

Scan your machine for infection using Malwarebytes Antimalware (http://www.malwarebytes.org/) and TrendMicro's Housecall (http://housecall.trendmicro.com/) online scanner. Remove all found infections. If a large number of infections are found, backup your data, wipe the hard drive, reinstall the operating system, and restore your data.
Change the password on your mail account. Use a strong password - see Microsoft Password Checker (https://www.microsoft.com/protect/fraud/passwords/checker.aspx).
Purchase and install security suite software - Kaspersky, McAfee, Symantec, etc. and then keep the signatures up-to-date.
If using Windows, configure Automatic Updates to run automatically or visit Windows Update manually after the second Tuesday of each month to install security updates to Windows.

You get a virus in your machine that sends it out, not really hacked, time to do many of the above or to take it to a professional