HELP! Win xp pro to pro won’t file share!!!

I have two pcs both running xp pro which I have had linked for some time now through the network cards direct. Everything has been fine regards file sharing etc.

I have just installed BT Business Broadband on the main machine and have lost the file sharing.

My main machine can access the internet and see the other machine and file share from it and to it. BUT the other machine cannot gain access to the main machine it syas something about access permissions and logon failure.

I am managing to share the internet connection tho!

Have shut down all firewalls and still the same.

HELP I’m completely lost.

Thanks in advance

Dan