When you installed Windows on the separate partition and then created an account with Administrator priveleges, the security identifier for that new account, SAM, is different than the Administrator account on the old Windows installation. So, Windows considers each a separate account and thus you are locked out. You need to enter the password for the old account to gain access, but because you installed it this way you are not provided with a password dialog box.
What you should have done is put that hard drive as slave on a different computer, and then browsed to the files and copied them to the new drive. When you place a hard drive as slave, it becomes an object owned by the Master drive, and thus access is allowed.
This type of task requires a great deal of technical experiance. I suggest you join a Windows Newsgroup, such as Windows XP Security and Administration. It is free to join, and once you do you can ask somebody to walk you through the steps to allow access to those files.
Here is a link to that group, but there are others as well, and it would be a good idea to locate one which meets this subject best. If you join one, you have joined all of these Technet Discussion Groups, so, you can easily ask your question at this one, and then ask if it is the correct group for this subject, and one of the experts there will let you know if it is, and if not, s/he will direct you to the correct group.
http://www.microsoft.com/technet/community/newsgroups/dgbrowser/en-us/default.mspx?dg=microsoft.public.windowsxp.security_admin
I am not an expert, but do have a moderate level of technical knowledge. I can build and maintain my own computers, run servers, etc, but I do not consider myself to be any type of expert. Thus, when I find an issue here that is beyond the scope of this group, I try to point the member in the right direction for assistance.
Please note that those who help less experienced users in these groups are experts in their own fields, and often if one walked into their offices would be charged a great deal of money for these same types of assistance, offered for free in the Usenet Groups. It is a wonderful way to receive quality help free of charge.
Keep in mind that everyone who assists others are doing so on their own time. When you ask a question please provide all the details of the issue at hand, including the OS, other hardware, all the steps taken thus far, and what exactly lead up to the issue and any error messages received. These kind folks can sometimes be a bit testy if group members don't provide clarified information, or if they have not taken basic steps to resolve the issue on their own. If one gets testy with you don't get testy back because if you do they may refuse to respond to further requests for help. I know that sounds a bit unfair, but consider what you get for free! lol
I hope you are able to retrieve your data, please have a nice day.