The drivers that are built into Windows XP and are automatically loaded when you connect your M-Audio MIDI controller are not "multi-client" drivers.

Multiple Clients

The M-Audio driver for USB MIDI controllers supports multiple applications opening both its MIDI In and Out ports simultaneously. This allows a user to have multiple applications running on the PC and to dynamically switch between them instead of having to shut down one application to free up the MIDI port so that another application can then use it. This is not possible when connecting the device without installing a driver.

Multiple devices

The M-Audio USB driver allows you to use Multiple devices of the same product family at the same time. Thus more than one keyboard (e.g. Keystation Pro 88 and Trigger Finger) can be connected to the same PC simultaneously and be recognized and used by the system. This allows a user to add drum trigger pads to his keyboard controller, or use multiple keyboards to control a Hammond B3 synthesizer supporting two keyboards, or for a user and one or more friends to jam using multiple keyboards connected to the same PC system. These multiple instances can be a mix of different or the same keyboards from the M-Audio USB MIDI controller product family. When multiple keyboards with the exact same product ID (PID; e.g. two Keystation 49e keyboards) are connected at the same time, the MIDI In and Out port names for these keyboards have a number appended so that the user can distinguish between them in the MIDI application software. This is also not possible without installing the driver provided by M-Audio.