Run hyperterm on com1.
With the null modem cable attached to com1:, on the other end you short together either the pins or socket # 2&3.
If it is pins the numbering is 1 2 3 4 5
6 7 8 9
If it is socker it is 5 4 3 2 1
9 8 7 6
A 25 pin/socket connector has the same order from pin 1 except it is 1-13 and 14-25.
You can use a paper clip as the jumper for a socket.
There are also handshake pins that are used with devices but hyperterm, IIRC, will communicate without them being active.
Normal loopback would short 4&5 and 6,8,&20, sometimes just 6&20.
If you will need to test often you might consider getting a 25s or 9s, whichever size the com port will take and permanently make a loopback plug.
Ed Fair
Give the wrong symptoms, get the wrong solutions.