Found another link that reminded me what I was looking for. It wasn't the PC itself. It's the cryptography that's probably on it. If you've got anything that supports encryption above 64-bit then it's classified as a weapon and export restricted.
The first commercially supported HTTPS server was classified as munitions
If you wanted a secure server you had to build it yourself and at that point it became illegal because you were using the patented algorithms that were only commercially available in one server. Apache via a particular vendor who licensed the cryptography libraries and at that point you had to sign all kinds of documents saying you wouldn't export it.
Back when RSA cryptography was prohibited from being exported from the US. I never tried to take it into and then out of the US, though. Didn't want that fight at the border!
I remember reading an account, then, of a mathematician who’d been scheduled to present a paper at a conference. The NSA intercepted him—because the maths he was speaking of could have been employed to devise (then) unbreakable encryptions—and forbade him to appear, threatening a lengthy prison sentence if he defied them.