I have run into a similar issue and it is the PC, not the Phast system (worked fine on Dell with XP s3, not on Toshiba with XP s3). The recommendation to use the USB adaptor seems to help in most cases, but it seems to depend on the chip set.
And, as for the off topic Phast-bashing...
It is too bad this thread becomes a Phast-bashing party, their systems worked, were a great value at the time, and have lasted longer than most. AMX has certainly done some dumb things, the biggest of which is not developing Phast (they had an issue with the 'not created here' ego delema), but they continue to support as they can given parts availability, etc. They have done much better, I have found, than many companies with old legacy product, including Crestron... Try telling a customer his system is not Y2K compliant (that is the year 2000 bug for the youngsters out there...) and that they should just buy a new Crestron system!!
Doing a crestron system 'back in the day' when Phast was current wouldn't have saved anyone, they have updated to newer equipment also, so it would be just as painful an upgrade, and unlike AMX, they won't service the old stuff (we were told last week to just buy a new part on e-bay because they didn't want to fix an old touch panel!!!). AMX has continued to repair almost all of the Phast equipment, and is very willing to help a dealer figure out a reasonable upgrade path when needed (as mentioned, Lutron lighting, etc.).
The biggest issue is that people bought systems from dealers that are either gone or have decided that they make more money selling a replacement system, so it is really the dealer that you work with that is the key. Here in Seattle a company called Home Technologies (which is now gone) was famous for insisting that they couldn't be repaired and then pushed full replacement (and now the scattering of their employees continues this profitable, though not very honest, approach). The fact is, old Phast systems are still maintainable and are as upgradable as much as any 10 year old electronics system can be. We continue to maintain all the systems we installed, upgrading them as needed, either within the system, or changing to AMX Netlinx (which supports the existing keypads, switchers, etc.). We now also service many of the systems installed by other integrators in the Seattle area (since most are gone or can't service them because they have lost access to AMX service).
Sorry for the soap box speech, it just annoys me when people blame the hardware instead of the dealer. If you bought a mercedes 10 years ago and then the dealer you bought it from went out of business, or became a BMW dealer, would you blame the car??? No, you would seek out a new Mercedes dealer. And if he said you had to replace the whole car because the tire was flat, you would tell him to &&*^ off and you would find another dealer that would help. Electronics is no different. Sure, eventually there are parts that are no longer made, so you upgrade parts or all of it and continue to enjoy the experience.