If you want to stay in the MS universe you could have a look at their Teams product. Is better than Skype/Lync. Not open source/internally hosted though. And

If you want to stay in the MS universe you could have a look at their Teams product. Is better than Skype/Lync. Not open source/internally hosted though. And dont feel too bad about office 2003. 4 years ago I worked for a large & well know company was using a more than decade old version of Lotus notes...