USB is just one of the many types of "ports" used by various computers - others are parallel, serial, MIDI, and in some cases firewire, also known as IEEE 1394, and some other names by some other manufacturers.
To get just one MIDI device connected to your computer, or a few more if "daisychained", all you need is a joystick (game) port on your sound card. There are cables available that convert the DB15 connector on the back of a standard sound card to MIDI in and out. This gives you only one MIDI port, and this port is usually only found on sound cards that get laughed at on serious music forums.
More advanced MIDI interfaces that can handle more MIDI devices without the timing delay that comes from "daisychaining" devices, normally use either a USB port or parallel port or serial port for connecting to the computer. The computer then needs to have the specific driver software installed for that particular MIDI interface, after which time however many MIDI ports the interface has, will be available to any windows program (one program at a time, usually)
You can probably get away with using a single MIDI connection, ALA soundcard type, for 3-4 MIDI devices, PROVIDING that you place the drum module FIRST in the chain of MIDI into the computer, and FIRST in the chain of MIDI out of the computer. This way, any timing lag will NOT be as obvious, since the drums will see the MIDI signal first, avoiding the lag of repeated loops thru multiple MIDI devices.
Not all MIDI devices are created equal when it comes to accurate timing, either - for example, the Roland XP=10 keyboard, while sharing voices with the more complex XP-50-60-80, is much sloppier responding to MIDI notes than the other three. This means that, as usuall, YMMV - you may notice severe timing lags, noticeable, or not even worth mentioning, just depending on the lag of each device.
Also, when daisychaining modules, unless one NEEDS to feed something to the next one, use the THRU connectors instead of the OUT connectors. This avoids some if not all the delay from unit to unit.
There is a Layla sound card by Echo, and a Delta 1010 by M Audio. Two different devices entirely. I'm not sure but I think the Layla has one MIDI port. Probably the Delta also. If so, you wouldn't need a USB port for either, as USB 1.1 is not fast enough to allow that many channels of audio simultaneously so both of those "cards" use a PCI card in the computer and an external "breakout " box for connections.
One hookup example could be: MIDI out of Delta, into Midi In of Drum machine - Midi OUT (NOT THRU) from drum machine to MIDI in of first keyboard - Midi THRU from first keyboard to MIDI IN of second keyboard, MIDI THRU of second keyboard to MIDI IN of third keyboard, etc - Then, MIDI OUT from first keyboard into MIDI IN of the Delta - Now, if you want to play drums from the pads on the drum machine, you can since its OUT goes to the IN on the first keyboard, and eventually into the computer. However, this will give quite a bit of delay and may not be satisfactory. The best way would be to play the drum machine with the first keyboard, closest to the input of the Delta. Then, the MIDI signal will go from keyboard to computer to drum machine.
All this confusion about hooking up what where and in what order is what prompts people like me to buy 8 x 8 MIDI interfaces. My system has a separate MIDI port for each controller, and separate ones for timing critical modules (3 different drum modules) I've beed doing it this way for the last 4-5 years, and would NEVER go back... Steve