I know I've updated my NAD Preamp/Processor's firmware over a USB-to-DB9 Serial adaptor under W7-64, and I also use my Dynaudio AIR Installer Software in "realtime" under W7 over the same USB-to-Serial adaptor, and also used the same adaptor with a W7-64 laptop to interface with my Sony DSP Car-Audio Unit from the 90's. All worked flawlessly...
Might be a risky proposition with a possiblt "fatal" result, but judging by my success I'd think it would work fine with a W7-64 compatible Serial (RS232) adaptor. Of course, YMMV...
You can also still find a few Motherboards that have native Serial Connections - but the options are thinning quickly with Sandy Bridge and this might not be an option for you. Some Motherboards actually have a Serial Controller onboard, but require the "MoBo header to PCI Bracket" breakout connector (won't see a DB-9 connector on the main I/O panel - it would be an extra PCI bracket that plugs into the MoBo in this scenario). Might be worth a peek...
RS232 is still VERY popular in Broadcast - We have oodles of it up here at work for the on-air automation system amung other "mission critical" things...
MADIface-XT+ARC / 3x HDSP MADI / ADI648
2x SSL Alphalink MADI AX
2x Multiface / 2x Digiface /2x ADI8