ORA-03113 is a tough one. There's stuff on MetaLink and a collection of cases here: http://home.clara.net/dwotton/dba/ora3113.htm

I did have genuine network probs with WinNT4 SP4 (TCP/IP stack handling probs), which gave ORA-03113, but they went away with SP5 - ancient history now. Worth checking if there are SP's for Win2003 that deal with IP probs.