Something to remember about XP is that XP is NT. NT5.1 to be exact. And with NT, you live and die by the MS Hardware Compatibility List (HCL) which is available via the MS web site. http://www.microsoft.com/whdc/hcl/search.mspx
In order for a vendor to have their product listed as "HCL Compliant" they must actually submit it and run a battery of tests on it. Passing the tests doesn't mean the product is faultless, but it does mean the product will perform substantially as expected, and without causing major system problems. (The odd ones can still happen, that's life.) Most NT-oldtimers won't touch a product unless it is on the list, or it is so "dumb" that it doesn't matter.
There are only two brands of USB-serial emulators on the list, one from IBM, the others from Keyspan. Either nobody else has bothered to submit one for certificaiton testing, or everything else that has been subnmitted has been unable to pass the tests. Lack of certification doesn't mean a product is bad--it just means you have no reason to expect it is good.<G>
USB-to-serial adapters have a bad reputation in general, probably because most of the drivers are poorly written, and some software still can't use them no matter how well they are written. On the other hand, serial ports supplied by a PC Card work 100% of the time, the ports are innately supported by the card specs. If you want to try a USB adapter, stick to one of the ones with an XP-compliant logo on the box. If you simply want one that works, robustly, with a proven track record, get a PC Card version.