There are acctually two ways of doing this, but I prefer the second solution.
The first way is to right-click Portal and turning off Steam cloud in the "updates" properties. Uninstall, reinstall, complete the game and you're done. NOTE: By completing the first three tasks you will be downloading a new, untouched copy of Portal, which is why you have to play it til the end. This will reset all records so make sure you do not have anything you'd like to save. The other solution takes care of this problem!
The second way is to type it all in the console. Here are all the bindings you need to make AFTER resetting keys to default bindings: 1 = slot1 2 = slot2 3 = slot3 4 = slot4 5 = slot5 6 = slot6 7 = slot7 c = impulse 50 g = phys_swap p = headtrack_reset_home_pos q = lastinv f = impulse 100 r = reload z = +zoom mwheelup = invprev mwheeldown = invnext
How to bind keys via the console: bind 1 slot1 bind c "impulse 50"
This solves it all. Oh, and please share these solutions to the rest of the world! :P