|"the problem I get is users plug in their USB flash thumb drive and the drive letter is the same as the map drives"|
That's not possible - the flash drive does not have a specific drive letter until it's plugged in. I assume you mean you can't get all the computers to use the same drive letter for the flash drive.
XP (and 2000, and Vista) will not a assign a drive lettter that is already being used, and it will not shift drive letters already assigned like ME and previous sometimes do when you connect or disconnect a drive.
You cannot change (at least, not by default, or easily) the drive letter of the partition Windows boots from, usually that's C, but you can change any other one above C to one not currently assigned. Sometimes you need to temporarily assign a different letter to free up a letter you want to use, then assign a different letter you want to use later.
When you plug in the flash drive, by default, the first available drive letter alphabetically, after C, is assigned to it, by Plug and Play and Windows. If there is a gap in the drive letters used, the flash drive will use the first one of those available, not the letter after the last letter used. If one has administrator rights, yes, the letter assigned to the flash drive can be changed - that's fine if the flash drive is NEVER unplugged and Windows has not been reloaded without restoring that setting - but if the flash drive is EVER unplugged, it will be assigned the first available drive letter again when it is plugged in - the drive letter assignment isn't permanent unless all the computers always have the same first drive letter available, and the subject flash drive is the first removable drive you plug in.
Whoever has administrator rights could change the mapped drive letters of drives on the local network to ones higher than currently assigned on each and every computer, the higher letters you use the less likely they are to be needed to be changed in the future, but the only way I know of to get what you want to be able to work and have all flash drives use the same drive letter, all the computers would have to have the same number of hard drive partitions, optical drives, and mapped drives on the local network, assigned drive letters below or above the letter you want to use for the flash drive.
If every computer has a C and D partition on one hard drive, and only one hard drive, and one optical drive E, and mapped drive letters were changed to letters higher than F, then the flash drive would always be using F when it was plugged in.
Of course, if the subject flash drive were unplugged, if the user has more than one USB or firewire connected drive and plugs it in before the subject drive has been plugged back in, they would have to be unplugged whenever you need to plug in the flash drive you are wanting to be F 0 they are assigned the first available drive letter too..
If some computers have more than one optical drive, the drive letter for that or those could be made higher too by the administrator.
If some computers have two hard drive partitions, some have more, you could have C and D for the first two partitions, E for the first optical drive, assign drive letters higher than F for any additional hard drive partition, mapped drive, or additional optical drive, the the flash drive would always use F when plugged in.
The problem with re-assigning drive letters like that, other than the letters for the mapped drives on the network, is it is likely, at the very least, some existing shortcuts will then be invalid. You might have more problems than that.
It would probably be much simpler to NOT use a flash drive for what you want to do, if not all the computers have the same installed drive situation.
You could try posting about your problem on the Networking forum on this site - it may take longer to get answers, but whoever answers may have better things to suggest.