I'd prefer as is, as Pierre32 said it's nice to get glimpse of it's past life. Seeing what it was used for, Checking file dates to see when it was last used. Kind of think LGR does with his old PC's
but also depends on who your targeting the system at.
If your targeting newbie's having a pre configured system all ready to go with a fresh install of windows would be a good selling point. I'd say these are the guys more likely to buy a complete system as well
Sound and graphics don't have to be amazing they can always upgrade to something of their taste further down the track.
Can even include a few pre installed games that have gone freeware,
OLD NOT NEW List of Free Games
People with bit more knowledge are more likely to buy parts individually but can see a motherboard and case still been desirable.
If aiming your add wording towards them just leave as is. 1/2 the fun is upgrading, installing, etc, etc.