I can play a movie using XBMC at full HD. When I change the user account, and play the same movie with XBMC the playback is choppy. I have notice the same problem exist for all kind of media, full HD or not.
- What is the problem?
If the first account is the administrator, while the second is a limited account, that would explain the difference. XBMC can experience problems under a limited account.
As you remarked that both accounts are admin, then another possibility is that XBMC was installed using the first account. In that case, I quote from XBMC Windows 7 Media Center Integration 2.0 :
If that is the case, then the installation of XBMC uses by error the wrong registry hive. Instead of using HKEY_CURRENT_USER, it should have used HKEY_LOCAL_MACHINE. Therefore, when playing under the second account, the registry settings for XBMC are simply missing.
The solution would be to logon as the first user, use regedit to export the above settings to a .reg file, logout, logon as the second user and import this .reg file into the current account. Please note that the registry key cited above may not be correct (don't have XBMC to test myself).
Have you got differing sets of codecs installed or differing settings for each user perhaps?
In addition, Windows 7 defaults to MS codecs so might want to check out the below to confirm defaults are correct for each user: http://www.codecguide.com/windows7_preferred_filter_tweaker.htm
Are you logging out the first user, or just switching users? If it's the latter, your video card may still have the video buffered in memory, thus reducing performance for a second video even if run the same way under a different user. Another possibility is that resource-intensive programs are running under the previous user that is still logged in.
If the first user is fully logged out, likely culprits include scheduled virus or spyware scans (if the second user is infrequently used these tend to trigger right on login, and continue on next login if they didn't have time to finish--wiping out I/O speeds) and different services running on the different user accounts (you can check this through examining task manager graphs carefully under each user account).
If none of these are the culprits, you will likely need to provide more details about the user accounts such as when they were created, what the specific differences in installed software, etc. are between the user accounts, and where the loaded media are being stored (including file permissions details for those folders).
Edit: Another potential issue is that apparently XBMC has problems with multiple users trying to use the media database. I would make sure that the second user is an administrator and/or check the file permissions for the XBMC folder. Alternatively, you could set up different profiles for each user to prevent media database conflicts.