My desktop VGC-RB57GY is very eratic about recognizing my Cruzer flash 4GB. When I insert the flash drive it will continually report that it is an unknown device. Occasionally it does recognize it but only for a short while, the drive letter is still there, but it will be empty.
This happens on any of the USB ports you connect the flash drive into? How does it perform when plugged into other computers? At the very least, make sure the motherboard drivers for your desktop are updated.