File sharing prob between two PC's through router on XP Pro


I'm having a strange little network problem and was hoping someone could help me with it.

I have two PC's, both running XP Pro, connected through a Zyxel Prestige 600 ADSL router. Both can access the internet okay, and they can both see each other and browse each other's shared folders okay, but whilst one can grab files from the other, it doesn't work the other way around. So A can copy a file from B and paste it into it's own drive, but B cannot copy a file from A. Also, if A attempts to write a file to B's shared drive, this doesn't work either. What's strange is that the file does actually appear on B, but it is corrupted. Also, when the copy/paste takes place both PC's show an egg timer and after about 30 secs show the message 'Error copying file or folder - the specified network name is no longer available'. After this if they try to browse each other's files they show the same message again, and they also both lose internet access. Only a reboot gets connectivity back, but the file sharing problem remains.

I don't think this is a hardware issue but I'll show specs anyway:

PC A spec:

MSI NForce K7N420D BIOS 2.8
512MB Crucial PC2100 RAM
Netgear FA310TX (onboard NIC has never worked and is disabled in the BIOS)

PC B spec:

Intel D875PBZ (latest BIOS)
P4 3GHz
512MB Corsair 3200 RAM
9700 Pro
Dual Seagate RAID HDD
Onboard NIC (has worked fine upto now)

I'm pretty sure this is some kind of sharing problem but I've checked that all folders are completely shared and can be modified by other PC's. Both machines have client for MS networks, file and printer sharing, QoS and TCP/IP ticked in network settings. Both are using specific (not auto) TCP settings, which are identical except for the IP addresses which are .2 and .3, with the router being .1. Also, the alternate DNS server addresses are slightly different. One ends in 33.42 and the other ends in 32.42. Both have NetBIOS enabled.

One thing worth noting is that the problem seems to have started since I flashed the NForce from BIOS 2.4 to 2.8 the other day. I can't see how this would make any difference but thought I'd add it anyway. After the flash I set up the BIOS the same as it was before, although I don't think there's anything in there that could cause this prob.

Anyway, sorry to waffle on. This prob has really stumped me.  Thanks for any ideas/help. :-)
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You can use a cross-over cable between the 2 systems and rule out the ADSL stuff.  Just assign IP addresses and respectively.
MungsteinAuthor Commented:
Thanks for the tip. I eventually found out what the problem was. It was indeed the BIOS. Having fiddled around for the better part of a day I finally decided to flash back to the old version and whaddya know; a fully functioning network. :-)
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