Right click the file that you want to take ownership of and select properties. At the top select the Security tab, select advanced options.
There should be an ownership tab (from memory) and you can take ownership from here. Apply the changes and and close the window.
Re-select Properties and Security. On the first page select Edit Permissions and the Add. Type in your user account name and it should certify the action, check the Full Control box below and Apply the changes.
That should do the job. I have recited this from memory, if it does not work I'll check again or find a link for you.
Good luck.