... without a fresh Windows install?

Is this possible? I want to add another HD for a backup and possibly upgrade size in future.

It currently has 1x 250Gb drive. Advent T9003 (father-in-law)

In most cases, no. RAID has to be setup in the BIOS and then in the OS, typically during install. I personally would not attempt it with XP. If RAID was absolutely essential, I would back up all my data, then start with a fresh install on the RAID array.

Note that RAID1, once considered a valuable method to ensure data integrity, is no longer very popular - except perhaps in a server environment.

Thanks very much for your reply.

I ask because i lost all my data/files due to a virus with a Dell Dimension 9100 that was Raid 0 (unkwnowing).

What do you suggest for the best? How are modern PC's setup? Just using an external backup drive?

Well having RAID1 would not have helped you with a virus. A RAID1 is a mirrored arrayed, which means anything on drive 1 is exactly imaged (mirrored) on drive2. So the virus would have been there too. RAID1 is only to help in the event of drive failure, not data failure.

External drive, image backup to a DVD, or to another computer is how most backups are done.