Microsoft disabled the "Users And Groups" MMC snap-in in XP Home & Vista Home. The only interface left is the crippled "User Accounts" in Control Panel, that gives you much less functionality.
I offer you two cool alternatives...
1. Hidden User Account Manager UserPasswords2
Use this command in Start>Run: %windir%\system32\control.exe userpasswords2
it will show a hidden User Account Manager that will allow you to do more stuff such as renaming your
real user name (not the display-name) and change group membership.
To add this tool permanently to the Control Panel, use this tweak.
2. NT4 User Manager works in XP!
On Windows XP you can still use the old User Manager tool from NT4. Download and use this tool to have a complete control over User Accounts, Groups, Password Resets and more.