First things to do: 1. Open Task Manager to see what is running 2. Run your anti virus and antispyware programs for a complete scan 3. Tell us what the "yellow light" indicates as most desktops have a green power light and a red drive light - or are you referring to the CD/DVD drive light? In the case of the latter being on check the drive has no disk in it.
UK's best mobile network 2016: best 3G, best 4G, best mobile networks revealed