So, instead of just grabbing an old favorite for your Windows 7 box, check out Microsoft's listing of Windows 7 anti-virus partners for companies that have already done some work in making sure their programs will work with Windows 7.
You can also check specific programs for Windows 7 compatibility with the Windows 7 Compatibility Center. As you search through this very useful site, you're going to quickly see that only the most recent versions of programs are certified to work with Windows 7 and that many of them are blessed only to work with 32-bit Windows 7 systems. So, if you're running Windows 7 on a newer, 64-bit system, like those powered by Intel Core 2 or Pentium Dual Core processors, you're likely to need to look harder for compatibile anti-viral software.
And, regardless of what anti-virus program you finally end up using, remember to always keep your operating system and applications up-to-date with the newest patches and to avoid clicking on links in dodgy e-mails. From Windows 1.0 to Windows 7, Windows remains an insecure system, and you have to go to extra trouble to make sure you're safe when you're using it.