If you have two installs, you should have two Windows folders on the same partition. Or did you install one on a different partition?
As for accessing data files on the first install...from the second install...that's easy. You can even access personal files, all you have to do is take ownership of them from the second install.
How to take ownership of a file or folder in Windows XP - http://support.microsoft.com/?kbid=308421
But you will not be able to "merge" both installs and you will not be able to move programs from one to the other. You can move favorites, WAB, etc. but no programs which are already installed.
If you go that route (moving to the second install) and you have two Windows folders...the way to get rid of the first install is to boot into the second and delete the Windows folder of the first. You should also edit the boot.ini file to reflect removal of the first.
I've done this
and I just chose to do a repair install of XP because I wanted to make sure that the boot files were in proper order. It was overkill on my part, but I was comfortable with doing that, took all of about 35 minutes.