The chart at Wiki should help with the differences click here.
The XP programs you're successfully running at the moment on Ultimate are most probably not running in 'XP mode'. To be running them in XP mode you should have:
1. Downloaded &/or manually installed Windows Virtual PC click here & used it to create a virtual hard disk, etc
2. Prior to this, checked if your CPU natively supports hardware virtualisation technology (HVT). Even if it does, sometimes it may need need to be manually enabled in the BIOS. A tool to quickly check if your CPU supports this feature - SecurAble click here