From Windows Vista Secrets 2nd edition:
Retail versions of Windows Vista must be activated within 30 days. Otherwise, the system slips into an annoying state in which it notifies you, every 60 minutes, that the system must be activated. Still, the 30-day grace period is useful, especially if you're just testing some things and want to make sure that your new install is working properly before you lock things down and tie your one Product Key to this particular PC.
That said, sometimes 30 days isn't enough. And if you want to extend this grace period, I've got some good news: Thanks to a barely documented feature aimed at Microsoft's corporate customers, it's actually possible to extend the activation grace period up to a total of 120 days. You just have to be a bit vigilant.
The key to extending the grace period is a command line program in Windows Vista called Software Licensing Manager (SLMGR), which is actually a VBScript script named slmgr.vbs. (It can be found in C:\Windows\System32 by default.) Using this script with the -rearm parameter, you can reset (or, in Software Licensing Manager lingo, "re-arm") Vista's 30 day activation grace period. This effectively resets the clock on the activation grace period back to a full 30 days whenever you run it.
The problem is, you can only successfully run this script three times. So it's theoretically possible to re-arm product activation's grace period to a total of 120 days (30 days of initial grace period plus three additional 30-day grace periods). That said, even the most careful of users will likely want to rearm the grace period with a few days remaining each time. But we're still looking at over 100 days of non-activated Windows Vista usage.
You can view your current grace period in the System window. To view this window, open the Start Menu, right-click the Computer icon, and choose Properties. The bottom section of this window, Windows activation, will display how many days you have until the grace period ends and will provide a link to activate Windows now.
Here's how to re-arm the Windows Vista product activation grace period.
1. Open the Start Menu and type cmd in Start Menu search.
2. Right-click the cmd shortcut that appears and choose Run as Administrator from the pop-up menu that appears. Windows Vista's command line window appears.
3. Type the following text in the command line window and tap Enter when complete:
slmgr.vbs -rearm
When the command is successful run, a Windows Script Host window will appear, noting, "Command completed successfully. Please restart the system for the changes to take effect."
4. Click OK to close the Windows Script Host window and then restart the PC. When you reboot, reload the System window. The grace period has been reset to 30 days. Voila!