It sounds like the PC is not on the same IP range as the TC and in that case it cannot connect to it, whereas perhaps the Mac could.
Just try.
open windows explorer and type in the address bar.
\\tcname or \\tcipaddress.
If it fails you will need to do the following.
Connect TC directly to the PC by ethernet.. No other network connections on computer or TC. press reset and hold it for 10sec until the front LED flashes rapidly.
This will not affect the backup only the TC setup. And you will have opportunity to save the previous profile, but that is not essential.
Once the utility has access to the TC.
1. Name the TC and wireless short names, no spaces pure alphanumeric.. The standard airport names are not SMB compliant.
2. Go to the disks page in manual setup and file sharing.. Put in the right workgroup, generally WORKGROUP.
3. It will make life much easier to use guest access with read and write permissions.
4. Go to the internet tab and in connection sharing, select off bridged mode.
Now update the TC. Turn off the pc... reconnect the TC back into the network..
You should now be able to access the TC .. open windows explorer and type in the address bar.
\\tcname or \\tcipaddress.
The airport utility should still be able to discover the IP of the TC once bridged if you don't know where it is.. You may have double NAT at the moment which prevents you finding it.
