If you are referring to hard drive partitions and you have multiple drives
then there is a hard way that involves re-doing the partitions.

Letter Assigner has been mentioned in this place as a possibility:
http://www.v72735.f2s.com/LetAssig/index.html

If you are referring to installed Windows devices (such as CD or ZIP drive)
there may be an option in the device properties to assign a drive letter.

The DOS SUBST command can often be used for specific purposes.
--
Jeff Richards
MS MVP W95/W98