see this link:
Windows XP, Vista, 7, Tips and Tricks
here's an excerpt:
1). Locate the file, folder, or drive which you want to take ownership in windows explorer.
2). Right click on file, folder, or drive and select 'Properties' from Context Menu.
3). Click on 'Security' tab.
4). Click on the 'Advanced' button.
5). Click on the 'Owner' tab.
6). Click on the 'Edit' Button and select a user from the list.
7). Click Ok to apply.
8). Place a check in the box 'Replace owner on subcontainers and objects'.
9). Click Ok when the Windows Security Prompt is displayed.
10. Now click Ok again to exist from Properties window.