udp packets necessary for wifi connection

Port 1900 is normally used for SSDP.

It is part of UPnP and will be included in XP as standard. Your Win 98SE machine does not have it has standard and it will only be included if you have perhaps included the "ICS Client" from XP. You could try disabling the UPnp Service on your XP machine and see if it will connect to the AP with port 1900 blocked. I would guess, not certain, that port 54605 is used as an alternative for M-SEARCH messages. "SEARCHPORT.UPNP.ORG OPTIONAL. If a device does not send the SEARCHPORT.UPNP.ORG header field, it MUST respond to unicast M-SEARCH messages on port 1900. Only if port 1900 is unavailable MAY a device select a different port to respond to unicast M-SEARCH messages. If a device sends the SEARCHPORT.UPNP.ORG header field, its field value MUST be an ASCII encoded integer, decimal, without leading zeros (leading zeroes, if present, MUST be ignored by the recipient), in the range 49152-65535 (RFC 4340). The device MUST respond to unicast M-SEARCH messages that are sent to the advertised port."

Port 1900 is uPNP, it shouldn't be required in most cases unless you're using uPNP, but I've seen weirder things.