If you log on by using a standard user account in Windows Vista, you cannot modify user environment variables in the System Properties dialog box.
For example, if you try to access the System Properties dialog box by clicking Advanced system settings in the System item in Control Panel, you are prompted for administrator account credentials. If you type the credentials for an administrator account, the user environment variables that you can access are for that administrator account only.
This issue occurs because of increased security in Windows Vista. The method that you must use to modify user environment variables in Windows Vista differs from earlier versions of Microsoft Windows.
To resolve this issue, modify the user environment variables by using the User Accounts item in Control Panel. To do this, follow these steps:
Click Start, type Accounts in the Start search box, and then click User Accounts under Programs.
If you are prompted for an administrator password or for a confirmation, type the password, or click Allow.
In the User Accounts dialog box, click Change my environment variables under Tasks.
Make the changes that you want to the user environment variables for your user account, and then click OK.