How do i make a cable modem from a different Service provider work on my computer?

Depends on modem but usually you can just switch the username and password:

First of all disconnect your modem from the power

Then go to Control Panel > Network Connections
Select your connection with a right click and select properties
Highlight Internet Protocol (TCP/IP) and press the properties button
In the new window select Use the following IP Address and fill the details below to your own:
IP Address: 192.168.100.10
Subnet Mask: 255.255.255.0
Default Gateway: 192.168.100.1

OK to all the windows

Now connect up your modem with the Power and Ethernet only, no need for the cable feed!

Stage 2
Now start HyperTerminal (if you don’t have HyperTerminal you can get a version of it from https://www.hilgraeve.com)

Make a new connection with the following settings:Connect using: (TCP/IP Winsock)
Host Address: 192.168.100.1
Port Number: 23

There will be a warning message, simply press Enter
It will then start scanning frequency, simply copy and paste the below into HyperTerminal:

cd cm_hal
scan_stop
cd

Now to change the password

Type:
cd non in the HyperTerminal then press Enter

Type:
cd msg in the HyperTerminal then press Enter

-----NOTICE
You may be asked for the original username and password during the next step, if you do not know what it is it is likely to be admin for the username and infinite for the password (NO CAPS, CASE SENSETIVE)
-----------------

Type: (Where it it says USERNAMEHERE replace with desired username)
user_name USERNAMEHERE in the HyperTerminal then press Enter

Type: (Where it it says PASSWORDHERE replace with desired password)
password PASSWORDHERE in the HyperTerminal then press Enter

Type:
Write in the HyperTerminal then press Enter

Stage 3
Once completed close HyperTerminal and power down the modem, go back to Internet Protocol (TCP/IP) Properties as we did in Stage 1, set them back to Obtain IP Address Automatically and the connect the modem up and power it on.

Completed, your password has now been changed

You have to call your local service provider and aquire there service and they will have to acivate the connection.