Sheldon says he has to install Windows 98 updates on the old computer that hasn't been used in years. But that's not possible now, as Microsoft ended support for Windows 98 in 2006, which happens to be over a year before the show even began.
Leonard says that Sheldon's spot only exists because he (Leonard) bought the couch. However, before Leonard moved in, Sheldon had a lawn chair in the same place and it was still his spot. Leonard just placed the couch so one end was where Sheldon's spot already was. Even if Leonard hadn't bought the couch, Sheldon would still have his spot.