Windows

This is because when you installed XP again, it creates a new SID (Security Identifier). Since this is randomly created, it doesn't match up to your old one. To resolve this, do the following:

1. Right click on your folder and select properties.
2. Select the Security Tab.
3. Click Advanced.
4. Select the Owner Tab.
5. Select your username and check Replace Owner on...
6. Click OK then wait for finish, and click OK again.

Note: If you cannot see the security tab, do this:
1. With a folder open, click tools and select folder options.
2. Click on the View Tab
3. Scroll to the bottom and deselect Use Simple File Sharing.
4. Click Apply, and then Click Apply to All Folders.
5. Click OK.

You should now have access to your old files.

Hope this helps.

have you tried logging on using the Administrator account, once in that account navigate around in My Computer, and change the access rights on the files and folders!
Hope this helps!