Hi,
I would like to unlock my iPhone ver. 2.0.1 by using windows vista operating system. I want to use my iPhone outside US as the lock period agreement from AT&T is now expired. I heard that PwnageTool is available for 2.0.1 version that can help me to unlock my iPhone. If my assumption is wrong, pls. suggest some genuine free unlock software. Thanks in advance.

Pwnagetool is mac only. It has always been mac only.
A quick search would have answered your question.

You obviously have an iPhone 3G - do you have a specific reason to stay on iOS 2.0.1?

The best option for you is a controlled upgrade to iOS 3.0, then jailbreak with redsn0w 0.9.2 and unlock with ultrasn0w.

As long as you avoid loading iOS 4.1 or 4.2.1 on your iPhone you won't lose unlock.
My advice, stay with iOS 3.x but understand that by going to iOS 3.x you can never go back to iOS 2.x again.