I've always wondered about this, if this is a myth or if there's a certain explanation to it... But does a computer really get any notable slower and gets a reduce of "lifetime" when installing something on it? I just cannot see how, if someone would be so nice to explain why. Well the SSD/harddrive might get a minimal lifetime reduction, but is this something really to care about? Or should someone optimally care about not installing any programs at all, if not necessary?
Let say I use Windows 7, a new computer and install a couple of programs like Adobe Photoshop and such. Why would the computer become slower?