My computer has a problem installing software after downloading the setup. The software I installed before the problem also stopped working. I performed a clean boot but it didn’t work.

Whenever I try to install Skype, it says it has problem connecting to download server. In the other hand, in the case of Windows Movie Maker, it said error: 0x80078007b and source: D3DX9, "unknown error occured". Could this be a computer virus? Being a college student, I really need to get thing up and running before the semester begins. I will be glad to receive any advice. Please help. Thanks in advance.