Sorry, I tried to delete my post which originally contained full details, forgetting that it could be useful to others.
The issue was: I loaded Windows 7 on Parallels on my new iMac and got it to read my XP machine fine, but next time I switched on my iMac, Windows Explorer was no longer reading the XP machine. I couldn't figure out why, hence my post, but then I realised that all I had to do was map the network drive, and it was fine.
