If your PC is connected to the Internet, there’s nothing stopping you from beginning to fold immediately. Point your browser at https://foldingathome.org/#downloads and choose your operating system. From there, install the client, and set up an account. Perhaps join a team, which pools the research carried out by multiple donors and turns it into a kind of competition between teams as to who can process the most work units. There’s a Maximum PC team, naturally: number 11108.
The minimum specs are very low, in the Pentium 4 and Windows XP range, but the more cores you can give it, the happier it will be, and only a limited number of projects are available for 32-bit systems. Once connected, you’ll find the client lurking in your system tray.