Your bst bet woudl of been to hire a IT guy off the bat and had presetup Os installs with permissions allready defined for IE in the install and then just deploy the OS across the network. But thats only if this was a perfect world lol Since you guys messed around and did not hire a proper IT guy, you will need to depend on third party software or vendor as a cheap alterative. The best thing to do would be overhual the network and restructure it properly.
FYI, at some point you will need a network overhual and restructure it completely as your business grows if you want it done correctly. AS another alterative, get a CISCO router and define filters from the router. However, thats not a secure method but works.
Edit:
Here I wanna explain a proper network in a nutshell for you quickly. First line of defense is the router (use CISCO) this protects you primarly from harmful incoming traffic. Next is the server, this protects you from outgoing traffic primarly and has a number of other functions. The last line of defense is the users machine and secruity setup by default. This is a very very basic network structure diagram and in many ways incomplete. It also requires a skilled Engineer such as my self to set all this up. To do a job correctly, you will need to spend around 20k or more. Including labor, server cost and Misc. expenses. The longer you prolong the overhual, the larger this expense will become. Microsoft charges Lic. per seat. Its going to get pricey if you don't overhual soon. Just a tip.