If your Laptop connects to the internet (presumably by USB Modem?) and you have enabled "Internet Connection Sharing" on that connection, *WINDOWS* will configure the network adapter used to connect to the Desktop PC with the address 192.168.0.1 and you *SHOULD NOT* configure the address yourself even using 192.168.0.1
When ICS has been enabled on the Laptop's internet connection, it will act as a DHCP server and the client PC (Desktop) should get its IP address automatically from the host PC (Laptop).
I have instructions for setting up an "Ad Hoc" wireless network with Internet Connection Sharing at click here
One other thing, if you have the Zone Alarm firewall running in the *HOST* computer, you must reduce the Internet Zone Security level from High to Medium or ICS traffic will be blocked.