Depends what you mean by 'backed up to an external hard drive'. If you mean simply copying an entire XP drive folder by folder and file by file, then you cannot have any problem - just copy back what you want. It will not be drive C: any more, remember, but will have a new drive letter. Also excepting that emails, for one example, will be in a format not necessarily understood by any Win 7 email program, and possibly also stored in places you are not aware of.
On the other hand, you may have made a 'drive image' on the external device, using one of several drive image packages, and now you DO have a problem. Many or most paid-for drive imagers have the ability to allow browsing and copying of the data inside the drive image, but mostly the free versions do not. Your solution to that is to recover the backup-up drive - hopefully onto a different HDD or partition to where you have since installed Win 7. From that recovered copy of the original XP drive, you can do copies as if you had done a straight copy as described above. It will not be drive C: any more, remember, but will have a new drive letter.
If the above explanations do not help you, you must give more detail of what you actually did.
