A PLC is a controller than can plug into a PC, and is supported by 3rd part software. Depending on your environment, there are some free options out there, such as inhomefre (windows):
The PLC will sell contain timers, so you do not need to leave your PC on.
A PLM is just a modem, it itself does not contain timers. Products such as our EZSrver work with the modem:
The PLM is not nearly as well supported by third part PC applications.
Your X10 PLC should coexist, however both the PLC and our EZServer support X10, so it would not seem to be necessary to retain your X10 PLC.