Nokia doesn't make any "microPC's", unless you mean the N800 which is more of a PDA than anything else. The Nokia does not have video out, so for presentaions it won't work.

The UX280/380 from Sony is a true PC, with all the power you need for a presentation. Battery life is always going to be an issue on these devices (they usually call them UMPC's) but most brands offer an extended battery.

At the moment my personal favorite is the Samsung Q1Ultra, though the upcoming HTC Shift will be a REAL winner.

If all you want is powerpoint you could also consider a small PDA with a VGA/presentation card. You won't get anything snazzy in Powerpoint but for a basic presentation they'll be fine.