First of all Windows 7 needs for booting 3 things:
- Windows MBR (or any MBR that loads active PBR)
- Windows 7 PartitionBootRecord on active partition
- Windows 7 bootmgr and \boot folder (with BCD inside) on active partition
With any partition manager ensure there is one partition on first disk marked ACTIVE.
( if you have a small 100MB partition called "SYSTEM RESERVED" it should be marked as active )
If you can still boot to XP you can try "Dual-boot repair" tool
The tool has a button "Automatic repair". Click it and confirm after that the tool will make your system boot to Windows 7 by default.
You should arrange multibooting so that Windows 7 boot manager has control of the multi-booting.
Windows MBR on first disk, Windows 7 boot files on fist disk.
Windows 7 boot manager can boot XP and Ubuntu.
You should have some basic understanding how Win7 is booting and what files are needed.
The same for XP.
The same for Ubuntu.
After that you should understand how Win7 boot manager can boot XP.
And how Win7 can boot Ubuntu/Linux based operating systems.
If you do not have at least basic understanding of how every single operating system boots
troubles with multi booting in the future can be expected to happen again.
Multibooting is not trivial and Windows 8 is comming...