Rather than a solution this is just from my past experience when using WinXP Laptop with USB port, LabV6.1 and NI-Visa for serial comms.
I used a USB to serial converter cable to link to the unit under test. The cable supplier sent a CD which did not include WinXP driver but said to download it from their website. On loading this I got a MS warning "Not Digitally Signed" and amongest other warnings..."May disable system...". On supplier advice I proceeded anyway. On running my test it would run OK for a while then suddenly crash showing blank screen which I later found had the term "Blue Screen of Death" (sounds horrible) from which I had to reboot the PC. Because I'd had partial successes with the test I persisted and used diagnostic tools like MAX, MS Device Manager (said device OK), Hardware Compatability Tester (HCT.exe) which showed some errors; forwarded my problem to MS and posted the results to the supplier who contacted the manufacturer with no help saying they'd not had any problems. All in all unlike your route my solution was changing the OS of 4 test laptops from XP to 2K and used the same suppliers CD which had the 2K driver. This was JUST MY EXPERIENCE from which some ideas may crop-up - I know of people working with different USB application systems and software on XP but have not had problems.
