How in the world can I change a nic from 100mbit full-duplex to 10mbit half-duplex?

The comp has slackware 8.0 and the nic is 3c905c-TX

From what I understand of man ifconfig,the correct command fo 10mbit would be "ifconfig eth0 media 10baseT", but that only gets me this error: "port: SIOCSIFMAP: Operation not supported" I know the nic has support for 10mibt, so what am I doing wrong here?

And for the half duplex bit I havent found anything

The data rate and duplex settings are negotiated by the card automatically when connected. However, I know there can be times when a slower connection would be preferred, for example 100 Mbit equipment but 10 Mbit cable. The man page states that some drivers may not support the option. From the message you posted, that was the response "Operation not supported".

If you are experiencing problems with the NIC, I know that early kernel support drivers were very flaky. If your kernel is 2.4.18 or 19 then I know it is stable. I think the drivers in 2.4.4 and 7 were flaky. Not sure when the problems were corrected. We stayed with the 905B cards or switched to Intel EtherExpress cards. If you are not running a recent kernel try upgrading your kernel.

You might also investigate if the the card has EEPROM capability. You might be able to use the 3 COM software and program the card to only configure for 10Mb/half instead of Auto. I do not know if the 905c has the feature or not.

Good luck.

Well one things for sure.
If you are trying to configure it remotely, ifconfig eth0 down isn't the greatest idea ... dammit