|What is DV ?|
Do you mean a DVD disk ?
"Unfortunately, I accidentally left the usb drive in the bus."
So the subject usb drive has been lost forever ?
"a. If I just directly cut & paste the video from DV to USB drive (I did not save in c: and D: drive), can I retrieve the video file from my computer (Windows xp version)now?"
When did you, or will you, do that ?
Back then or now ?
If you copied it from a DVD disk to the USB drive it was never on the hard drive, unless you played the video on the hard drive.
"b. If I watched the video 3days ago in my computer, is there any possibility to retrieve the video file from my computer?"
Extremely unlikely, unless you have not used the computer at all since then, if a copy of the file was even stored at all on the same hard drive partition the Windows folder is installed on. The Windows page file has probably over-written the data area where the video file was.