It's not clear where you are now at. Are you trying to dual boot? Did it roll back ?
If it did - or if you are able to boot into XP - look on the 7 partition in ~bt\Sources\Rollback , or .~bt\Sources\Panther check the last entries in the setupact and setuperr logs. They should say what stopped it.
If you are stuck and unable to boot into anything right now, you could try this:
Boot the 7 dvd ( select Repair My Computer instead of Install Now) - it might attempt automatic startup repair. If not, after it has searched for installations , click Next and select startup repair. If it says it was successful - then restart, and if necessary, select windows 7 from the boot menu.
If that is not successful - and you can't boot into anything:[ and if you are trying to dual boot and still have XP installed]
Boot 7 dvd to Repair My Computer - this time, select the command prompt, type
bootsect /nt52 all
then press enter . close cmd prompt. Restart. You should then be able to boot into XP.