This is probably due to the network authentication that XP defaults to. This has been documented on this list several times.
You can fix on local XP machine or on your iSeries. There is a PTF for V6R1 - see here http://archive.midrange.com/midrange-l/201105/msg00518.html
Or under Control Panel, Administrative tools, Local Security Policy, Local Policies, Security Options, Network security: LAN Manager authentication level, select send LM & NTLM - use NTLMv2 session security if negotiated.