[dba-Tech] Missing Network Service user

I'm trying to install DotNetNuke in Windows 7 and SQL 2008. I've configured
IIS and created a DNN database, but now I'm stumbling on a new problem. The
tut says right-click on the directory and select properties and its security
tab, then choose Network Service from the list of users. It isn't in the
list. I don't know why. How can I make it appear? Alternatively, can I use
another one instead, such as SYSTEM?

TIA,
Arthur