matlamont
We use Sage Accounts software for wages etc which is installed on the server (although only 1 client actually uses the software)
I do not fully agree with Stefanos comment/interpretation.
Depending on the particular software, you can install software into a samba share (ie an ibay) on your sme server (ie where the software requires to be run on a Windows workstation). It will be accessible to all networked Windows workstation users who have access rights to that ibay (configured by ibay/group membership).
Some multi user software may not work correctly (eg sharing or indexing errors etc) unless the software coders have developed it to work in a Linux networking environment, some software may need settings to be tweaked in sme eg oplocks. As you only have one user requiring concurrent access, then you should not have multi user concurrent access issues.
If the software requires a specific server type eg Windows SBS2003 or whichever operating system to install & run on, then of course you will not be able to use it on sme.
You really need to check with the software makers.
Your bigger problem, as has been pointed out, is the use of MS Exchange, which requires a MS Windows server. You can use the one physical server to run sme & Windows in a virtual environment, but you will have to face a serious learning curve in order to implement this if you have never used a virtual server before. For starters look here
http://wiki.contribs.org/Vmware