Cannot uninstall ISA Server 2006. Error right off the bat

Before I took over the role as IT at my company, the previous guy had implemented ISA Server 2006 to test if the company could use. They ended up scrapping the idea, but he left it installed on the server. I need to implement Ms Business Contact Manager on that server and in order to share the databases, I need to remove ISA's hold on the ICS service. I tried add/remove for getting rid of ISA, but it errors out saying, "The installation source for this product is not available. Verify that the source exists and that you can access it." I get this almost immediately and it's at the "Core Components" phase. I do NOT have the original CD and am working off of an iso left on the server where the old guy did the original install. I've also tried an ISO from my company's FTP. I get the same error for both. I also get the error when I try to run the install again. If I run the same iso install on another server, everything is just peachy.

This is on a Windows 2003 R2 box running full SQL Server 2005.