So, it's Saturday night and I'm at work, so that's a bad start. I've got one Windows 7 PC out of 25 that's having a network problem. It can connect to the Internet fine (I'm typing this on it as I speak) but it can only intermittently see the Samba file share we use in the office. If I change the port this PC is connected to on the switch, the situation reverses - I can access the share perfectly but the Internet connection stops working.
I've turned off the Windows firewall, and that's not helped. I've tried different IP addresses and workgroups, all to no avail. I've tried different network cables - no joy. I've been through about ten ports on the switch but they either allow Internet or share access.
Earlier today this PC was connected to a small switch with four others that could access the Internet and the share just fine, even though they were all using the same port back at the "main" switch.
Even stranger still is that when the problem occurred earlier in the day I swapped out the PC for a brand new one and it continued having the same problem!
I get a feeling that this is a problem with some obscure fix that simply hasn't occurred to me, and someone out there will be able to help.
Okay, I've "fixed" it by swapping out the master switch for a different one and now it works fine.
I'd still appreciate knowing what could have caused the problem, though, as with my (fairly limited, admittedly) knowledge of how a network works this simply shouldn't have been possible.