I try to keep most of the boxes that I've received over the years, still got the boxes for my ZX Spectrum, Amiga 1200, Sega Megadrive etc.. I've literally run out of room though (almost to the point of being accused of being a hoarder).
When I built my PC back in june I kept most of the boxes although if I had duplicate boxes (e.g. GPU's etc...) I kept one & threw the other one out