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If they are 'blank', then the question is, are they formatted? or, are they even working at all?
Generally, if even an unformatted flash drive is inserted into a Windows computer's USB port, this should happen:
1. the computer should make an audible signal - a sort of 'klunk-lunk' sound - to signify that it has sensed a device plugged into the USB port.
2. it may say either 'preparing USB drive for use' or 'installing drivers' or some such.
and 3. it may possibly offer to, or insist upon, formatting the flash drive.
If none of those things are happening, then either
A. your computer (esp. the Win XP one, which may be quite old!) has either no means of sensing the flash drives, or else the USB port you have been using may be defective. USB ports are not all equal, and sometimes they don't work. Try another USB port (and don't depend on getting anything working on a Windows XP computer, for Windows XP is no longer supported since April 2014 and is pretty much obsolescent if still marginally workable.)
B. you have defective flash drives, period... either burned out, or destroyed by static discharge, etc.
If you have a known working flash drive, testing both the computer and the USB port(s) with it would be appropriate, before coming to any conclusions on this problem.
Last resort: look in
Start - Computer
or in Win XP, Start - My Computer
after plugging in the flash drive. See if it shows up there as an extra drive #, e.g. F: G: H: or further drive letters. If it does, right-click on it, and select 'Format' (after being sure you are not formatting the C: or D: !!!! which is a grave mistake, almost always.)