There are a lot of reasons when you just don't want to activate after 30 days. Maybe you didn't set your clock, and now after you changed it, Vista is whining. Maybe all your new hardware didn't arrive yet, and you do not want to have to mess around with getting it reactivated. Whatever the reason, Microsoft allows you to extend activation time up to 180 days!
* Open an Admin Command Prompt (right click on cmd, and select run as Administrator)
* type 'slmgr -rearm'
* now you have another 30 days, repeat until 180 days
Note: This has been noted that it may have been disabled after certain patches were applied to Vista. Please try at your own risk.