In the Computer Management console I don't see Local Users and Groups under the System Tools.
How do I fix that?

Hope you find some valuable info here

How To Use Computer Management in Windows XP (http://support.microsoft.com/kb/308423)

Step-by-Step Guide to the Microsoft Management Console (http://technet.microsoft.com/en-us/library/bb742442.aspx)

Using the Computer Management Console's Local Users and Groups snap-in in Windows Server 2003 | Servers and Storage | TechRepublic.com (http://blogs.techrepublic.com.com/datacenter/?p=295)

How do i fix that?

If you are using Windows XP Home Edition, you don't - that's one of the differences from XP Professional.