Hard Drive Security when changing OS

I experienced an odd thing when I moved to VISTA and wondered if anyone knew of a solution. I had an old PC running on XP. I built a new PC recently and installed VISTA on a new HD. I am done with the old PC components so I thought I'd just take the old HD and plug it into the new PC and use it as a 2nd HD. I also wanted to backup some of the files. However, when I pluged the old HD into the new system ,it didn't let me access the files (I believe because it was formatted with an OS that was not the main OS of the new system).

I tried everything including booting up in safe mode but I still cannot get access to the files on the old HD. Is this possible to do or do I need to backup everything on the old system to CD/DVD then copy to new HD that way?

Any insight is appreciated. Perhaps I was naive thinking a HD would be plug-n-play.

Can you be more specific? I think I have tried this but am not sure to what exactly you are referring (sorry...probably a novice compared to you)