3 devices, 1 router: Only 1 device hacked/ hijacked?

Hello all,

my husband and I use a splitter to connect our PC's to the internet. Our tablet connects to the modem directly via WiFi (darn handy invention ).

My husband's PC started to have issues, crashing and rebooting beyond belief. Tried to repair Windows, and then it got stuck at rebooting at the flash screen.
Took it to shop to figure out what was going on; we suspected possible HD failure (it's several years old and has a lot of use behind it). Guy called in, and states that the PC had a bunch of rootkits applied to it.

Funny, since we have internet security program (both of us have the same), and my PC is clean as a whistle.

I checked some online pages to see for ways to find out if a PC has been hacked / hijacked, but nothing of those 'tell-tale signs' applied: No unexpected programs, odd mails, redirecting or anything. So it makes me very weary and uncertain about the guy in the shop; almost feels like a rip-off.

We have the issue sorted, so that's not the problem anymore; however the thing that bothers me is this:

How can it be, that only 1 PC that shares an IP with the other, is having those issues? Wouldn't it be logical, if both our PC's would have problems?
May sound weird of me think that simple, but in my logic we would both have some issues.

Thank you in advance to anyone who can clarify this a bit in (preferably) novice-language

 

Is there any way, we can get proof that the PC was really damaged / hacked / hijacked, besides just the 'word' of the shop keeper?
I mean, usually repairs or something leave a log or trace behind, not sure how this stuff works.
Not a novice to PC's but am alienated by unknown issues xD
I know I am curious, just like to learn