If legacy usb drivers are enabled by your BIOS, then you should always be able to make it recognize the usb device as a generic mouse over PS/2 or serial in a way that works even at the install menu for Windows 7 before the other drivers you need for the SMBus and so forth are installed for you during the set up process. Even if it doesn't have that option on the BIOS for some reason, then you can still go through set up and install Windows 7 using the keyboard until you're able to get to the point that it can install generic drivers for you after it's installed and booted. If you don't have a recognized mouse yet, then one helpful tip you can use right away is to go to Control Panel and the Accessibility Options to turn on Mouse Keys (this was available from either Windows 2000 SP4 or XP on out) and use the arrow keys to control the mouse (the number 5 at the center lets you "left click" icons, while you can use either the *, -, or / keys to toggle to right-click and back (it's been a while, but one of those keys does it and it will only take a second to see which one toggles for you by clicking any area on the desktop to see. If a box pops up with context menu options, you've right-clicked. If not, then left-clicked.
If there aren't driver options available for it for you to download, then you can use a tool like what is at drp.su to identify and install the drivers you need (even offline) instantly after the DEV ID is found for it. If it's a really rare mouse driver, it may have to download it manually if you have network access from that system. If for some reason you don't have drivers for your ethernet or wifi adapter yet to connect to the net, you can use another computer (or even a mobile phone) to download it if another connection and/or computer system isn't available. Since most modern mobile phones have their own device driver for your computer when you connect it via usb, you can instantly connect and use your phone as if it were a usb flash drive, and copy the driver for your mouse or the program to install it from your phone to your PC that way.
If you have an automatic device driver updater, it will identify and install the correct mouse driver for you without having to go to the Device Manager to install or correct that manually.