The answer is no. More interestingly, even 20+ year old software was not necessarily disclosed in a way that meets the legal requirements for "prior art", so yo

The answer is no. More interestingly, even 20+ year old software was not necessarily disclosed in a way that meets the legal requirements for "prior art", so you can be sued for using it. I believe exceptions for prior usage are one of the things being messed with in the current "patent reform" legislation, though, IIRC, they're being influenced in a negative direction disappointingly and unsurprisingly.