All computers can contribute to Folding@home. However, if the computer is too slow (e.g. wasn’t built in the last 5 years or so), the computer might not be fast enough to make the deadlines of typical work units. A Pentium 4 or newer equivalent computer (with SSE) is able to complete work units before they expire.
Folding part time (less than 24 hours a day) will increase the minimum system requirements to make the deadlines. In this case, it is up to the donor to determine how many hours a day are required to fold and still meet the deadlines with each computer.