go to Start->Run type inetmgr and press OK. If you get an IIS configuration screen. It is installed, otherwise it isn’t.
You can also check ControlPanel->Add Remove Programs, Click Add Remove Windows Components and look for IIS in the list of installed components.
EDIT
To Reinstall IIS.
Control Panel -> Add Remove Programs -> Click Add Remove Windows Components
Uncheck IIS box
Click next and follow prompts to UnInstall IIS.
Insert your windows disc into the appropriate drive.
Control Panel -> Add Remove Programs -> Click Add Remove Windows Components
Check IIS box
Click next and follow prompts to Install IIS.