For the Compaq, my first concern is that the power supply may not have enough extra capacity for the CD-RW drive.
Also, 64MB is not enough RAM for running a CD-RW drive. 196MB - 256MB would be a good idea.
Next, check for available slots, and make sure that they are not using 'propritary' cables.
Since you can find 56K modems for under $50 (for a full modem), it makes little sense to try and find the drivers for an old modem.
There are driver links at www.modemhelp.org
and www.driverguide.com
For the ASUS, you could add a USB 2.0 plug in card.
There is a header on the motherboard that you can use to add USB connectors to an unused expansion slot.
Check out the manual for the pinout.
Make sure that there is a W2K driver available for the modem.
The USB modems that I have have ONLY W98 drivers.
I would at least double the RAM for W2K, you should consider maxing out the RAM the motherboard can take.
Chas