USB to serial converters are readily available. Avoid the really cheap ones as their drivers can be a bit nasty. Go for a brand you trust.
Alternatively you may want to consider an Ethernet - RS232 converter if you have a network available. Usually these come with COM port emulation drivers, but you don't necessarily need you to load these drivers if your communications software can talk direct to a TCP device.
You will most likely need to convert from 9 pin to 25 pin, and it sounds like you will also need a gender changer adapter. You may or may not need a straight through or null-modem cable. Perhaps even a custom cable altogether. To find out exactly what connections you need to make requires the CNC machine's pinout diagram or a little detective work with a voltmeter or a breakout box.
There are many options for software, depending upon what you want to do. The cheapest option is hyperterminal (built in with Windows) but it isn't really meant for this job, the terminology isn't self-explanatory for CNC.
I have written a CNC communications package which you may want to consider using. (www.iworkshop.co.uk) There is plenty more information about RS232 communications on my website if you look for and download the helpfile.
Hope this helps