Perhaps I'm just being a fool thinking I can connect an 'old' G5 to a machine running Win7
Not at all, your problem is simply a lack of familiarity with the technology, not foolishness.
It doesn't matter how old the device is, if it talks TCP/IP it can talk to anything else using TCP/IP*.
At least at a connection level.
The problems tend to occur higher up the stack, when you try to to get a new application to talk to older kit that may not support that app.
However, SMB / CIFS (which is what we are talking about here) is an established standard, adequately supported by both devices.
You will get there, if you persevere. It's perfectly possible. I can't help with specifics because I haven't worked on a Mac for almost 20 years!
SD
* for the experts, I know I'm ignoring IPv6 here...