Because of the built-in VB script in vista we can increase the trail period by 30 days but it will work only three times i.e, u can extend it for 120 days and the best part is it's not violation of Windows End-User license agreement.
The following process of extending trail period using script is known as REARMING.
Step 1:-Goto Start>All programs>Accessories>Command Prompt
Step 2:-Don't just open command prompt,right click on it and select Run as Administrator.It will take u to this location C:\Windows\system32 .If not,then manually enter the location in ur cmd
Step 3:-Now just type the below script and press enter
cscript slmgr.vbs -rearm
Step 4:-It will prompt for restart.After restart u will see ur trail period extended by 30 days enjoy have a nice day :)