There are 2 basic ways to do it.
1. Purchase a null modem cable ( about $12us). Make sure it is a DB25female to DB25female. Setup Direct Cable Connection
( found in settings-controlpanel-addnew software-windows setup-communications )
Connect the two machines together. Start one machine as host, one machine as client.
You must have microsoft networking loaded, and file sharing turned on.
This is probably the cheapest, but 2 problems exist, one, you're limited to the length of the nullmodem cable 25ft or less, and you must start the direct connection each time you boot, either manually, or through windows scheduler.
The next step up is to create an ethernet lan. You will need a small 2-4 port hub, about $30 - $40, 2 Network Interface Cards ( NIC) around $12 each, and 2 patch cables, anywhere from $2 - $30 depending on length.
Make sure all devices above have RJ-45 connectors to simplify things. The patch cables connect the PC's to the Hub, and can be anywhere from 3 ft to 300 ft in length, so you and mom don't have to share the same room.
Install the NIC's in the PCs, load TCP/IP, Netbeui, and Microsoft Networking. Again, turn on file and print sharing. Connect the PC's to the Hub, and away ya go...The beauty of this arrangement is it also allows you to share a DSL or Cable Modem if you get one, so you don't need more than 1 internet access account.