Setting and unsetting an environment variable in Windows

Using the command line

Set an environment variable

1. Run > cmd

2. Type:

set <variable name>=<value>

Example:

set JAVA_HOME=C:\Program Files\Java\jdk1.6.0_21

3. Optional: Double-check by viewing the value of the environment variable by typing:

set

Unset an environment variable

1. Run > cmd

2. Type:

set <variable> =

Example: 
set JAVA_HOME=

3. Optional: Double-check by viewing the value of the environment variable by typing:

set

Using the GUI

Set an environment variable

1. Right click My Computer > Properties > Advanced system settings

2. Click Environment Variables... button (bottom right of window)

3. Click New...

4. Enter Variable Name and Variable Value

Unset an environment variable

1. Right click My Computer > Properties > Advanced system settings

2. Click Environment Variables... button

3. Click the variable you want to unset

4. Click the Delete button