There are many factors that contribute to the maximum read speed for any USB device.
For Flash memory devices, it is possible for the port to operate faster than the write speed.
Date transfer rates are dependant on system type and configuration.
I assume that your PC has on-board USB 1.1 ports, and a PCI USB 2.0 card.
Your PCI buss clock speed and actual PCI slot used can impact the data transfer rate.
While the PCI buss is rated for 33MHz, MANY systems clock the PCI buss slower.
Shared resources will slow down the data rate to a given PCI device.
It is general practice to avoid using the PCI slot that shares with the ISA buss on the motherboard.
The amount of CPU and motherboard cache RAM will also impact data transfer rates.
The OS and installed service packs, along with any software that runs in the background WILL affect the data transfer rate.
The speed difference may be as simple as a difference in Anti-Virus software configuration.
If you want a moredefinitive answer, you will need to provide more system details.
Motherboard and CPU,
USB controller IC make and model
Make and model of your USB drive.
Chas