PS> $env:windir
C:\Windows
PS> $env:ProgramFiles
C:\Program Files

PS> "My computer name $env:COMPUTERNAME"
My computer name MYHome-test-01

PS> ls env:
Name              Value
----              -----
ALLUSERSPROFILE        C:\ProgramData
APPDATA            C:\User\sv-test\Home\AppData\Roaming
CommonProgramFiles       C:\Program Files\Common Files
COMPUTERNAME          MYHome-test-01
ComSpec            C:\Windows\system32\cmd.exe
FP_NO_HOST_CHECK        NO
HOMEDRIVE           C:
HOMEPATH            Users\v-test\Home

PS> $env:TestVar1="This is my environment variable"
PS> $env:TestVar2="Hollow, environment variable"
PS> ls env:Test*

Name              Value
----              -----
TestVar1            This is my environment variable
TestVar2            Hollow, environment variable

PS> del env:windir
PS> $env:windir
PS>

PS> $env:OS
Windows_NT
PS> $env:OS="Redhat Linux"
PS> $env:OS
Redhat Linux

PS> md .myscript

  Directory:

Mode        LastWriteTime   Length Name
----        -------------   ------ ----
d----    2011/11/29   18:20      myscript

PS> cd .myscript
PSmyscript> "write-host 'Hollow , Powershell'" > hollow.ps1
PSmyscript> .hollow.ps1
Hollow , Powershell
PSmyscript> cd ..
PS> $env:Path+=";C:PowerShellmyscript"
PS> hollow.ps1
Hollow , Powershell
PS> hollow
Hollow , Powershell

PS> [environment]::SetEnvironmentvariable("Path", ";c:\powershellscript", "User")
PS> [environment]::GetEnvironmentvariable("Path", "User")
;c:\powershellscript