You forget / didn't update the Active Partition and move NTLDR over to it (the RAID), so you can boot from it.... and update the Registry entries for the GUID of the drives to 'switch' drive letters around. (Windows XP uses GUID for HDDs and tends to ignore the underlying hardware config).
May I suggest reinstalling Windows XP from scratch and 'porting' your applications over.
You can use 'R' for a Recovery Console, (not 'R' for Repair, different page of installation guide), when installing Windows XP, then run DISKPART to update your Active / Primary partitions, etc
For example on my own system:
4 x 300 GB HDDs - Maxtor Diamond Max 10 - in RAID-0 (really around 280 GB)
3 x Primary Partitions, each 16 GB for Windows XP, Windows XP x64, and '3rd Operating System'. These are on the fastest part (outside zone) of the HDDs, and RAID-0 array.
The first Primary Partition is marked Active, so it is the one the OS boots from
1 x 1069.9 GB Extented Partition, containing 4 logical drives, each 280 GB, (except the last which is 229.9 GB), this makes backups to external 300 GB (really 280 GB) HDDs much easier because all the data always fits.
When installing a '2nd Windows XP' installation, usually in another partition / drive, make sure to mark the partition active beforehand, otherwise it screws the drive letters around (they may be different in each 'installation' of your Operating Systems, which can become confusing).
Your old HDD was the drive with the system files, boot loader, etc on it, with Primary Partitions marked as Active and set as the Boot Devices / Drive order in the BIOS, your Windows installation may have been copied (highly unlikely) to your RAID-0, but the Boot drive doesn't change, and removing it will cause Windows to either not boot (Above - NTLDR missing) or fail at the login screen (it gets there, but can't login) because it uses GUIDs for all the drives, then assigns drive letters (not using the DOS method), and when it can't see the windows installation you can't login.
The easiest 'fix' from here it to just reinstall directly to the arrays 1st partition, after setting as active and boot drive in BIOS (Boot Order of devices, etc) it is less screwing around in the long run. You'll need to use 'F6' during and insert your SATA/RAID Driver diskette early on in the WinXP installation, or it either won't see the array, or risk destroying it (and all the data on it).
Your mainboard should have come with a SATA/RAID Driver diskette to install the drivers 'while' the OS is installed, thus permitting people to both, install directly to & easily boot from (less screwing around with GUID in Registry, etc) their RAID arrays.