Problem fixed
What I did was to change the passwords on the XP PC to something else then back again, it now works ok. Still not sure how that could have fixed it as both PC's had identical accounts and passwords on them that happily access the local PC but not the other PC on the network.
Thought I ought to post the fix as it may help others. I would also like to thank those who offerd advice. I have definatly decided networking is a black art though fun.