I have multple Networks and multiple domain, how can I setup the scan with different networks

3 Spice ups

multiple networks YES (via the SETTINGS>Network Scan settings)

also can use remote scanners on those separate networks and have it send to the central install

http://community.spiceworks.com/how_to/show/283

however,

multiple domains NO (NOT with same central install, can only scan and sync to one AD install )

You can setup your scan with multiple networks by following this article.

http://community.spiceworks.com/help/Adding_New_Devices#Range_Settings

Each network will have one or more of its own scan range(s). You will create a user (domain level if necessary) that is a local admin on each workstation you want to scan. This account does not need to be a domain admin, but a local admin on the computer under the scan. You will need this admin account to collect the full inventory. If your network is large or the other networks are beyond a WAN you may want to setup remote collectors (Spiceworks in remote collector mode) at the remote sites for scan efficiency and to limit the amount of WAN bandwidth consumed during the scan. The remote collectors will forward all scan information to the central server. Almost no data is stored on the remote collector.

1 Spice up

Is this one forest? If so, you should be able to use an enterprise admin account with access to all for scanning Windows based devices. Alternatively, you could use SNMP across all the networks. You did not give details, so it is difficult to advise you, but you should be able to use remote collectors for each domain and have them update a centralized database- however you would not be able to use AD integration unless as I first asked this is a forest with multiple domains.