I had a store do a bunch of stuff on the computer last year while I was away. He put on copy of XP Pro, which I found out quickly is pirated (no updates, loads of viruses etc)
I have just bought copy of XP Home and need to instal it, preferably without wiping hard drive.
Can this be done, simplest method please. Thanks

Nope, not to my knowledge without wiping out the partition first.

You can load the other OS for the time being, get an external USB pen drive or other external storage device and attempt to copy any data you need first.

Also maybe the User Profile Backup/Restore is still working, if so you should be able to backup easily this way too.