I suspect that your files were made Private in your user account settings in XP. This is the bane of computer techs.
You will have to take ownership of the files on your old hard disk as an administrator on your Vista Computer. To do this, you will have to right-click on the drive letter assigned to the old drive and select the Sharing and Security Option, then Under the Advanced Section you can select the Owner tab and take ownership of the files on the drive. I don't have a Vista machine handy to check, but in XP if you are using the Home version, you can only find the Security and Owner options while in safe mode. XP Pro can perform this task in Normal Mode. This may also apply to Vista. Taking ownership of the files and folders on your old drive will then allow you to access and copy those files you need.
Let me know if you have trouble with this and I will get myself in front of a Vista computer to verify the proper steps (if they differ from XP).