Can someone help me troubleshoot my iTunes?

Factual Questions
1

I have a fifth-generation video iPod, used with a Windows PC. It worked great for about a year, then it suddenly didn’t. The timing seemed to coincide with a period when I was borrowing some music from a friend, whose sources tend to be, let’s say, inexpensive. As in free. Of course that might be a coincidence, but that’s about when it happened.

My iTunes doesn’t recognize the device at all now. I can’t even find it in Explorer. Yes, I’ve tried it with other cables. I’ve un/reinstalled iTunes several times. Oddnesses that make no logical sense me, though:
–it errors out when I tried to load it from someone else’s iTunes; different computer entirely, which works fine with their own iPod.
–it works perfectly when reformatted to a Mac.
–other iPods plugged into my original PC also go unrecognized. This would suggest that the problem is native to the PC, but then see the first point above.

Does anyone have the knowledge/time/patience/charity to help me figure this out?

It would be vastly appreciated, I assure you.

2

Have you ran spyware and anti-virus programs to disinfect your PC? If not, then you need to do so. Are you running Spyware Doctor? If so, then uninstall that as apparently, there’s an incompatibility between it an iTunes, all of a sudden. You might also try uninstalling the current version and installing an older version of iTunes. I had a similar problem and rolling back to an older version and uninstalling Spyware Doctor seems to have fixed the problems.