Read only attribute over XP network

I have 2 computers running XP pro SP2, peer to peer network. One computer has just been upgraded (hardware) to P4 and programs preloaded, data copied across.
All OK on that machine, but other machine can no longer access files on first machine with full rights, ie files read only.
Sharing options all setup OK, can’t see anything in permissions, but then maybe I wouldn’t recognise it if it bit me.

Anyhelp appreciated.

TIA