Questions here as the original statement is unclear.
Have you actually completed the migration to Win7 and are now trying to back up the finished drive? Or are you trying to backup the WinXP drive prior to migration?
How big is the hard drive you are trying to back up, and how much data is there on it?
As for the backup routine supplied with WinXP, that was a deprecated program that didn't work very well. In fact it usually didn't work........Microsoft only put it on the XP setup disk as an extra so you could extract files from Win2000 backups. It was never intended to be used for backing up XP systems
If you are backing up prior to the migration, then DON'T.
Instead, COPY the files you want to save to the external drive, and copy them back again afterwards. You cannot trust a "backup" to save the files as you have no way of checking them, especially as the backup software is not cross platform compatible