Hi, I work in IT so i might be able to help
Something to check might be is the USB pen drive using a same drive letter as another device for example a DVD drive. It might be worth trying to change the drive letter.
How to change the drive letter in Windows XP for an external USB stick or hard drive
Can you see the Pen Drive in the device manager at all?
What type of Pen Drive is it?
The Optiplex is a business machine, does this belong to you personally? Does it belong to a corporate network?