I carry HTC HD2, with window operating system and i wanna upgrade it to window 7 or android.I  've heard there is some crack(free version).Can anyone give me some idea, which link is trust worthy, and how do i do it? 

There are several Android Build for HTC-Hd2. You need to figure out which one is best for you. Go to http://forum.xda-developers.com/forumdisplay.php?f=735 for the list of available Android. Its a series of hit and trials. Keep on trying until you are satisfied. Most of them work just fine. Just download the Android file and copy the File folder to your SD and run from there. Just follow the instructions and you will be good to go. Going through the youtube tutorials can also give some ideas regarding the installation and available Android File. Good Luck.

Thanks much, that was really helpful. I have one more question, if i am not satisfied with android can i switch it back to my same windows OS with all the content and setting i had?

Just restart your phone and u'll be using window again. You can even Dual boot window and android. You need to install android loader software for this.

restart like, factory reset?

No just turn off and start again. As simple as that. Even if you install android, window doesn't get deleted. Android is just running from the SD card so Window will still be your primary operating system.