Overall, you should not need an install disk. The OS detects the modem and has built in support, or should. But in the case that you do need it (rarely) you get it from the internet provider, call them.
If the modem has a standrad network port, just connect it to your pc via lan cable, it will then allocate an i.p to your pc and your good to go.
If its by USB, then you will need a driver installed, if its an older modem, W7 may not support it.