I don't know Vista - yet; not likely to for a long time either...
However I'm surmising that file accesss etc. is pretty much like that for the earlier NT style OS - NT/W2K//XP?
This in that if you can't access files/folders from Vista that were made under XP (in your case) you may be able to take ownership of it/them when logged-on as Admin (in Vista)? Did you try this approach?
If you use an XP system (with the problem drive attached somehow...) then this M$-KB will probably guide you through the access-path OK?
http://support.microsoft.com/defaul...
and this link is to Vista equivalent path...
http://windowshelp.microsoft.com/wi...
And if you trawl/search the M$-KB using:
How to take ownership of a file or folder in Windows Vista
as the string/search-term you will get the above "hit" and a few more besides...
This link has a few (freebie) download utils that may help here - if allse fais - in terms of gaining access to (some/all) files? Haven't used it/them so can't comment on effectiveness etc...
http://search.techrepublic.com.com/...
I think the M$-KB will do what's required in terms of accessing the files/folders?
Why you can't complete the re-installation of XP? Where are you trying to put it - not in the same partition as Vista presumably? If Vista is at all like XP it will get a little upset if it has to share its partition with another OS - unless its Vista (in this case). I have gotten two copies of XP in the same partition... but few manage to get XP to successfully/peacefully co-habit with any other OS...