Could be a faulty trackpad. They can be replaced. On many laptops' trackpads, ctrl+up/down on the right hand side of the pad is interpreted as zoom (e.g. in browser windows). If disabling the trackpad and using a mouse causes the problem to go away, there's your answer. Replacement track pads aren't too expensive on (e.g.) eBay, but it's not a DIY job unless you're pretty familiar with laptop disassembly.
