From Windows 7, right click on 'Computer' - Manage - Storage - Disk management. Now you should be able to see a partition that has no drive letter assigned to. Right click on this partition and click Change Driver Letter and Paths.
Then click Add and choose Assign the following driver letter. Here you can choose any letter that you want or leave the same shown by default and click Ok and close all windows.
Now go to ‘computer’. You should be able to see the XP or Vista partition here. Now you can access anything saved on your XP or Vista partition from Window 7.