Setup Windows Server

We have a small business network which we were running using a Linksys SR2024 unmanaged router, and a Linksys WAG160N 4 port modem/wireless router to get our ADSL2 internet. The person who setup the system previously set it up with an additional Netgear wireless router so that some of the laptops that connect to our network can receive wireless access. Not sure if this can be done away with and use just the one modem/wireless router. The server is a HP DL 180 Proliant G5 server with 1 NIC, but we have installed an additional card which has 2 NICs. The server has 6GB RAM, a quad core xeon processor (E5430) 2.66 Ghz, and 6x300GB HD. However, the server only came with Windows 2003 SBS server so it can't take full use of the resources. The person who set it up, got me to setup the free VMware and we currently have 4 virtual servers running, the Windows SBS 2003 running as domain controller, a Linux file server, a Linux gateway server and a Zimbra Mail server. We have huge issues with our internet connection being slow/dropping out both for people getting wireless access, and for those who are AD members and are connected via ethernet cable to the router. People don't seem to be getting IP address automatically, and we have had to manually put them in which is a huge issue for laptops connecting wirelessly, as they use different setting when out on the road visiting clients. The mail server is causing issues as we have purchased Trend Worry Free Security but it can't be installed to monitor our mail server as it isn't using exchange, and so we are getting hundreds of SPAM email everyday to each mail account. We also need to purchase a backup software to allow us to backup exchange/mail server, file server, sql databases, intranet etc but haven't purchased one yet as system is running satisfactory. However, we do have 2 1.5TB drives ready to use for the backup. Need to work this out urgently. I have a reasonable knowledge of system and hardware and can help with the installation changes.