will the 8300 modem work with XP

SBG8300 will provide 2.4 and 5G WIFI network so you can look for 2.4 network on your XP to connect to your home network or you can try changing the security mode on the modem interface to get connected by following the steps Gateway > Connection > Wi-Fi > Edit 2.4 GHz> Security mode> WPA2/PSK(AES)> Click Save. If still same issue persist please do contact the device manufacturer to check the compatibility.

I'm not trying to connect via wi-fi. The direct cable connection is supposed to be up to 250 download but ever since I installed this 3.1 docsis modem the speed averages 32 download

XP as in windows XP? 

and you’re hardwiring the computer in? 

The SBG8300 shouldn’t impact that at all. If you’re having issues hardwiring it in, i’d be looking at your ethernet controller or xp install (as dang thats ancient)

Yes it's ancient but my previous Arris 3.0 modern worked fine.   No slow down.  Cox wants to blame my machine but that shouldn't all of a sudden cause an issue unless there's some incompatibility. 

Looking at the specs somewhere for the 8300 it said windows 7 and higher

Well, SBG8300 is compatible with all devices it wouldn't  affect any particular device when connected to the home network .