If you need to find the name of a PC and all you have is the IP address and you do not have access to Active Directory you can quickly look it up using the NBTSTAT command.
Open up Command Prompt and type:
nbtstat -a 192.168.1.1 (Replace the IP address with the appropriate one)
This command will also give you the MAC address if the computer.
Alternatively if you are in a domain you can use Nslookup: