Do you mean for PCs WITHOUT serial cards? Just about any USB/serial converter cable should work.
Not actually 100% true. From previous posts, I have noticed many have had problems with some of the Prolific chipset based adapters in Win7 and possibly earlier Win versions. For actual info, if anyone is interested, I would recommend searching through old postings on eHAM and QRZ to spot the trends of problematic adapters and ensure I avoided those types to increase the probability of success using USB to serial adapters, if indeed that is the question being asked here.
I'm still unclear as I stated earlier.