Activation requires the customer to connect to Microsoft, either over the Internet or the phone, to activate the operating system.
The Activation process automatically ties the computer to the newly installed operating system based on the machine's hardware.
If attempts are made to install the same copy of Windows XP on one or more machines, the new installation can't be activated. In addition, Windows XP has a 60-day limit to be activated. After the time limit, it won't work.
The XP cracks floating around the Internet claim to either disable the Activation feature or circumvent it.
One crack claims to reset the computer's internal clock so Windows XP constantly thinks the user has 60 days to register the software.
Another claims to disable the Activation feature entirely. The patch, which is 13 MB in size, purports to replace files that need to be activated with non-activated versions from the corporate edition.
Some of the hacks are fairly complicated, requiring file installations and edits to the Windows registry.
Windows XP comes in two versions, a Home and a Professional edition. Only the Professional edition appears to be available on the Internet.