Windows XP includes the Fast User Switching feature, which is available as long as your Windows XP computer is NOT joined to a domain.

Microsoft list the following information about Fast User Switching:

For Windows XP Home Edition:

The feature is always available.
The feature is enabled by default on computers with more than 64 MB of RAM.
The feature can be enabled or disabled in Control Panel / User Accounts / Change the way users log on or off.

For Windows XP Professional computers that are NOT joined to a domain:

The feature is enabled by default on computers with more than 64 MB of RAM.
The feature can be enabled or disabled in Control Panel / User Accounts / Change the way users log on or off.

NOTE: The feature may be disabled if you install a 3rd-party GINA (Msgina.dll).

If you have 128MB of RAM but you system has Shared Video Memory which reserves 8MB or more for use as video RAM, Fast User Switching will be disabled, because Fast User Switching actually requires 120MB of RAM.