The hack(trick) shown here may be small but by using this hack you can use win7 for approx one year without activation. So lets see the process..
Step-1: Go to Start ►All Programs ►Accessories, Now right click on commnd prompt and select "Run as administrator"
Step-2: Now enter the command given below
Now you will be promoted for a restart, so make a restart. Once you have been restarted your trial period will reset to 30 days again. You can use this commend for 3 times, so that you will get 120 days of trial period(30+30+30+30=120)slmgr -rearm
Step-3: Now here comes the real game, just navigate to Start ►Run and type "regedit". So that you will be navigated to Registry Editor.
Once you are once again navigate to the location given below
Step-4: Once you navigate you'll see a key called "skipRearm" on the right side of panel, Just change its value to "1"HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\SoftwareProtectionPlatform
That's it, by doing this trick you will be able to use the command "slmgr -rearm" another 8 times so that you will be able to get another 240 days of trial period which you can use with out activation.
By the whole thing you able to get 120+240=360 days of trial period.
Hope this helps you! pls pass your comments.