|suprisingly enough, XP (disdaining serial ports as too crude) allows conversation with serial ports where 98 and ME don't.|
assuming com1 (3F8 = data):
107 OUT DX,AL
the timing is a matter of experiment to get 10ms. (you might have to load CX instead of CL and do a CMP CX,0 : JNZ test to get longer times).
the voltage varies from about -8 to about 7 (15 volts diff). As Wizard remarked, the current is going to be low, so the type of relay is critical (as i'm sure you're aware of) The ns16550 is quite versatile and rugged driving 1489s but I haven't a clue how much current drain it will support for indefinite periods.
they make an off-the-shelf serial-port/relay interface that might be worth consideration, to handle current amplification. also (I'm sure) addressable versions.