Ubuntu is totally do-able in a corporate environment; I know from personal experience.
It's hard to say what will happen in your case without some specifics.
When you say 'network with Windows', can you be more specific? There are certain networking protocols that both should follow (DNS, DHCP, SMTP, etc). However, if you are using Microsoft Active Directory, for example, you may have some difficulties I have had outstanding success with Likewise by Centeris. Their motto is:
Making Linux, UNIX and Mac Systems First-Class Citizens in a Windows Environment™
If you don't know much about Linux and you are the IT department, I would recommend setting up some Linux machines at home, and then a pilot/test group of a few tech-savvy people at work. Canonical does offer support, as well as Dell, if you buy a Dell with Ubuntu.
As far as your accounting software, this, again is unclear without knowing specifics. I kinda doubt that they have a Linux client (This may not be obvious but if you get a .exe from them, you won't be able to install it without a hassle. You may be able to do so via Wine but, again, this is a case-by-case basis). If they do have a Linux client, you should consult their documentation for specific information about how this will work.
As far as Symantec, you will probably just be OK using ClamAV for anti-virus. (It's free!)
http://www.clamav.net
However, Symantec does offer SEPP for Linux:
Linux Operating Systems (32-bit and 64-bit versions)
* Red Hat Enterprise Linux 3.x, 4.x, 5.x
* SuSE Linux Enterprise (server/desktop) 9.x, 10.x
* Novell Open Enterprise Server (OES/OES2)
* VMWare ESX 2.5, 3.x
* Ubuntu 7.x, 8.x
* Debian 4.x
Whether or not you would need to purchase this or if it's already available to you will depend on your contract with Symantec, as far as I know.
Setting up some Linux machines first at home will be the first step on this journey. Doing this and using the computers for everyday tasks will help you start generating questions. And we're here for that. :-).