It sounds like E is also mapped to a path on C, likely via the SUBST command (a Windows command)
As for slowing down your PC, I can't think of a reason why that's related to MATLAB itself. It gets installed like any other program and should only be taking resources when it is started in the default installation. Or perhaps you are running low on disk space and the installation has brought you very close to having a full hard drive. But this is all speculation. I'd suggest you talk with people familiar with your system setup and troubleshoot it.
If it's slow while running MATLAB, it's possible that you need more RAM or you are running too many other programs. Look at the Task Manager to determine if you are using a lot of CPU or memory and your system has started using virtual memory, which is much slower than RAM.
In addition, MathWorks provides free installation help.