For Hosted Edition:
All your support staff and customers need to use Hot help Desk is a web browser. There's no plugins or other files to download
For Installable Edition:
Hot Help Desk is written in HTML and ASP.Net 2.0 + AJAX 1.0, which requires a machine running a Windows server OS (WinNT, Windows 2000, Windows 2003) and IIS (which is built in to all Windows server OS's). The .Net 2.0 framework is also required.