simpleContact's requirements are deliberately modest. You need a web server with PHP support and access to a MySQL database.
simpleContact Lite
PHP 4 and MySQL 4.0 are the oldest versions that I support for simpleContact Lite.
simpleContact Pro
With the release of version 2.1, which adds support for UTF-8 text encoding, the minimum requirements for simpleContact Pro have been revised. PHP 5 and MySQL 4.1 are now the earliest compatible versions.
Other requirements of simpleContact Lite and Pro
The built-in update check requires either fopen or cURL.
The "remember me" checkbox at login uses a cookie.