Question : Setup 1 computer on Lan to access a seperate Net connection then the rest.

We have recently been running into an issue where one of our users needs to regularly FTP files to a external server.
Whenever this user is FTPing out its sending at roughly our max upload speed(800Kb/s and we can not get a faster upload from our ISP) this seems to slowdown the network connection for everyone on the lan, to an unbearable speed or pages keep coming up as cannot be resolved.

We have 2 internet connections one that is connected to the Lan, another that is used for guests of the company thats physically seperate from the Lan.

The Users computer has 2 Ethernet ports, one currently in use for the Lan and the other one free and is using windows XP Pro.
I'm wondering what I would need to do to connect her 2nd port to the Guest internet and have her computer use that connection for internet yet still have access to the LAN and the ability to upload files from the lan out to the 2nd internet connection.

The user would have a direct connection from the 2nd Ethernet port to the Guest internet router.

Our Lan currently has DNS being handled by the primary domain controller and on the 1st Ethernet port the DNS points to the server, so I dont know if this could be problematic.

Answer : Setup 1 computer on Lan to access a seperate Net connection then the rest.

If your user were to configure the second NIC with an IP address in the second ISP router's address space, and that address space is different from your "internal" network (192.168.1.0/24 versus 192.168.2.0/24 for instance), your idea would work well. You will need to set a persistent route in your user's PC that points his 0.0.0.0 traffic out the second ISP with a metric of 1 to ensure that this works.

I'd use a static IP oin the second ISP network, to avoid potential changing route metric complications.

You have identified one primary issue you may have, DNS. But as long as your DC is resolving outside IPs for outside sites, you should be ok on that aspect since the PC will simply choose the best route regardless of where the name resolution comes from.

Your other potential risk is security, if that user's PC starts routing, or if that guest LAN is less well protected than the internal network, keep in mind that his PC can act as a gateway into your "inside" network from the "guest" network.

Does this answer your question?