List all hotfixes applied to windows server

The following command(s) work in both the 'regular' command prompt and PowerShell, also for both Windows Server 2003 and Windows Server 2008:

wmic qfe get Hotfixid

to find if a specific update was installed:

wmic qfe | find "123456"

It however does not provide any office updates (unfortunately), thought it's relatively easy now to just go to 'installed updates' and do a search for it... same as Windows updates, but the question specified it needed to be done from a command prompt.

Powershell only command:

Get-Hotfix -id kb123456

You can do it with Powershell:

Get-ChildItem -Path “HKLM:\SOFTWARE\Microsoft\Windows NT\CurrentVersion\HotFix”

PowerShell now includes the Get-HotFix command which is far easier to remember.