You can do this through Disk Management.
From Microsoft Knowledge Base Article 307844:
"To change an existing drive letter on a drive, on a
partition, or on a volume, follow these steps:
1. Log on as Administrator or as a member of the
Administrators group.
2. Click Start, click Control Panel, and then click
Performance and Maintenance.
3. Click Administrative Tools, double-click Computer
Management, and then click Disk Management in the left
pane.
4. Right-click the drive, the partition, the logical
drive, or the volume that you want to assign a drive
letter to, and then click Change Drive Letter and Paths.
5. Click Change.
6. Click Assign the following drive letter if it is not
already selected, click the drive letter that you want to
use, and then click OK.
7. Click Yes when you are prompted to confirm the drive
letter change.
The drive letter of the drive, the partition, or the
volume that you specified is changed, and the new drive
letter appears in the appropriate drive, partition, or
volume in the Disk Management tool."
