Motherboards can and do break down. For example, the capacitors on the boards can eventually leak over time, but it's not as if you're going to notice a slow progression in performance on that basis. Does the PC have problems turning on sometimes, or perhaps even failing POST? Sometimes these are indications something is afoot with capacitors, but a sure-fire way to check would be a visual inspection. A classic example would be a build-up of crud on any capacitors, which could also be accompanied by damage to the motherboard from leaking electrolytes.
However, without going too far down the line and pointing you in the wrong direction, it's difficult to provide more insight without really knowing your current set-up, i.e. hardware installed, OS, free disk space, etc.
"An ounce of prevention is worth a pound of cure" ~ Ben Franklin