You can download a free evaluation copy of SwissSys. The evaluation version does everything the registered version does, except that you will be limited to two rounds of pairing until you register.
When you purchase a registration, either as a first-time buyer or when upgrading from a previous version, you will receive a serial number that unlocks the evaluation version. The Demo runs in the full USCF + FIDE mode. If you purchase a FIDE-only registration the key you receive will lock out non-FIDE options, such as the USCF pairing engine, while enabling you to run full tournaments in FIDE mode. No need to re-install in either case.
Click on the link below. When prompted, select "Run" to install SwissSys. You may also elect to save the program and install directly from your computer later by double-clicking on the saved file.
Note: If you get a warning from MicroSoft's SmartScreen Filter, select "Run anyway", or "Actions | Run anyway".